(19) United States
`(12) Patent Application Publication (10) Pub. N0.: US 2008/0138593 A1
`Martinez
`(43) Pub. Date:
`Jun. 12, 2008
`
`US 20080138593A1
`
`(54) THIN FOAMED POLYETHYLENE SHEETS
`(76) Inventor:
`Felipe Martinez, Houston, TX
`(US)
`Correspondence Address:
`The DOW Chemlcal C°mPfmY
`lnfcenectual Property sectlon’ P‘O‘ BOX 1967
`Mldland’ MI 48641-1967
`(21) Appl. No.:
`10/560,732
`(22) PCT F 11 e d:
`Jun 30, 2004
`
`(86) PCT NOJ
`§ 371 (OX1),
`(2)’ (4) Date;
`
`PCT/Us04/21173
`
`Dec_ 15, 2005
`_
`_
`Related U‘s‘ Apphcatlon Data
`(60) Provisional application No. 60/485,292, ?led on Jul. 7,
`2003'
`
`_
`_
`_
`_
`Pubhcatlon Classl?catlon
`
`(51) Int, Cl,
`3323 27/32
`C08,] 9/00
`
`(200601)
`(200601)
`
`(52) US. Cl. ....................................... .. 428/220; 521/ 134
`
`ABSTRACT
`(57)
`The present invention relates to the use of particular blends of
`LLDPE and LDPE together With speci?c fabrication condi
`tions to make foamed sheets of thin gauge With MD tear
`properties similar to an equivalent gauge non-foamed sheet of
`the same composition. In particular, blends Which combine a
`high Ml LLDPE rich fraction With a 10W Ml branched LDPE
`minor fraction provide the adequate balance of mechanical
`strength of the polymer base With its melt strength, extensi
`bility and stress relaxation, alloWing a thin foam ?lm With
`MD tear strength Which is comparable to non foamed coun
`terparts of similar gauge and composition. The foamed ?lms
`of the present invention are thin (generally from about 1 to 10
`mils thick), and have an MD tear Strength Ofat least about 160
`grams for a 3 mil ?lm as measured by ASTM D 1922. The
`foamed ?lms of the present invention are preferably made at
`least in part from blends Which comprise from 10 to 95
`percent by Weight of LLDPE having a relatively high (1.5 to
`6) MI and from 5 to 90 percent of an LDPE resin having a
`relatively loW MI (0.5 to 2.0).
`
`Page 1 of 9
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`BOREALIS EXHIBIT 1070
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`

`
`Patent Application Publication
`
`Jun. 12, 2008 Sheet 1 0f 2
`
`US 2008/0138593 A1
`
`Figure l
`
`Foamed Films @ 20 percent Density Reduction
`Eimendorf Tear vs Gauge
`
`0 MD Tear
`In CD Tear
`a MD Tear 3 mii NF
`x.
`CD Tear 3 mil NF
`"Linear (MD Tear)
`“Linear (CD Tear)
`
`Film Geuge (mils)
`
`NF = Non Foarned
`‘
`
`1
`
`
`
`
`
`Eimendorf Tear (g) '
`
`Page 2 of 9
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`

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`Patent Application Publication
`
`Jun. 12, 2008 Sheet 2 0f 2
`
`US 2008/0138593 A1
`
`
`
`
`
`Melt Strength (cN)
`
`Figure 2
`
`o 100% Resin B‘
`a 30% Resin E |
`70% Resin E |
`x 82% Resin E’
`x 100% Resin E
`o 100% Resin A
`+ 100% Resin 6
`
`O‘
`
`i
`
`"
`
`100
`
`-
`
`200
`
`300
`
`Extensibility (mm/s)
`
`Page 3 of 9
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`

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`US 2008/0138593 A1
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`Jun. 12, 2008
`
`THIN FOAMED POLYETHYLENE SHEETS
`
`[0001] The present invention relates to thin foamed poly
`ethylene sheets, particularly those at gauges betWeen 1 and 10
`mils. These sheets have comparable MD tear strength as
`nonfoamed sheets of the same gauge and composition. The
`invention also relates to the process of making such foamed
`sheets.
`[0002] The thin sheets or ?lms of the present invention are
`used in many applications, particularly in bloWn-?lm appli
`cations including consumer trash bags, grocery bags, produce
`bags, pallet Wrap, food Wrap, liners, heavy duty bags, indus
`trial bags, consumer bags, shrink ?lms, labels, pouches for
`FFS packaging, tapes, stand-up pouches, lamination ?lms,
`protective ?lms, health and hygiene ?lm applications. Similar
`thin foamed ?lms can be made using cast ?lm and sheet
`extrusion lines, but these Will exhibit preferential orientation
`in the MD direction and hence Weaker properties. Foamed
`?lms can be made in the form of monolayer or coextruded
`?lms With multiple layers, Where one or more of the layers are
`foamed. These thin foamed ?lms can be further laminated to
`other substrates including, foil, paper, other plastics, or they
`can be post stretched in one or tWo directions for obtaining
`Wrinkled skin surface effects. In the polyole?n industry, there
`has been a general trend to produce neW high strength poly
`mer resins. These resins have alloWed ?lm producers to
`doWnguage their product Without sacri?cing ?lm strength or
`toughness. These thinner products have not been universally
`accepted hoWever, as the perception of ?lms of having a limp
`or ?imsy feel. Accordingly, it is desired to produce ?lms of
`greater thickness. It is not cost effective to simply use more
`resin to make a thicker sheet hoWever, because of the cost of
`additional raW material. It is generally knoWn that polyole?n
`resins can be foamed in order to produce a thicker ?lm With
`the same amount of resin. It is generally understood, hoWever
`that properties such as tensile strength, impact strength and
`elongation are related to density, and that the foaming process
`results in a product having less density and potential for Weak
`failure spots. Thus, prior ?lms or thin sheets made from
`foamed polyole?n material lacked adequate strength.
`[0003] High pressure LDPE resins have been used in foam
`ing applications due to their relatively high melt strength,
`strain hardening behavior and easy processing. HoWever,
`When making a foamed sheet at gauges betWeen 1 and 8 mils
`using conventional bloWn ?lm processes With these resins,
`excessive orientation results, Which in turns leads to very poor
`results in MD (machine direction) tear strength. Accordingly
`there is a need for thin ?lms of reduced density Which still
`exhibit acceptable physical properties, particularly MD tear
`strength.
`[0004] Some reported solutions to the problem of increas
`ing physical properties in thin foamed sheets include U. S. Pat.
`No. 4,657,811 and Us. Pat. No. 4,533,578 Which provide for
`coextrusion of unfoamed skin layers around the foamed layer.
`This method achieves the increase in tear strength at the
`expense of complexity of the ?lm structure and loWer overall
`density.
`[0005] It is also generally knoWn from Work With thicker
`foam sheets that crosslinking provides molecular ties and that
`these molecular ties enhance physical/mechanical properties
`such as tensile strength, tear strength, higher temperature
`resistance, etc. As discussed in “Foamed Films Find NeW
`Niches”, Plastics Technology Online, Jan H. Schut, February
`
`2002), crosslinking is also being investigated as a Way to
`improve mechanical support of thin foamed ?lms. Crosslink
`ing adds cost and complexity to the process, and results in
`material Which cannot be easily recycled, and is therefore is
`less than ideal solution.
`[0006] Yet another approach to improve physical properties
`is bi-orientation. As discussed in the Schut article mentioned
`above, traditional tenter frame bi-axial orientation for cast
`?lms are typically done in the semi-solid phase using a tWo
`step process (machine and transverse-direction orientation),
`usually ending in collapsing of the foam cells. Traditional
`BloWn ?lm process can achieve simultaneous orientation
`both in the machine direction and in the transverse direction,
`being able to apply up to 3:1 MD and 4:1 TD orientation
`levels, While the polymer is in the semi-molten state. Some
`neW orientation methods reported in the Schut article claim to
`be able to have apply a bi-axial orientation of 3 .5 : 1 in MD and
`4.5:1 TD While the polymer is in the solid state, Which gives
`even higher strength. In conventional (non-foamed) ?lm
`extrusion it is knoWn that the use of Linear LoW Density
`Polyethylene (LLDPE) resins, especially those having frac
`tional Melt Index (MI) and loWer density, helps to improve
`MD tear properties. It is generally believed that resins With
`loWer density and loWer MI (higher molecular Weight) pro
`duce better physical toughness. It is also knoWn that for
`conventional ?lms the use of high bloW-up ratio (BUR) in
`processing the resin provides balanced machine direction/
`cross (or “transverse”) direction orientation Which improves
`overall ?lm toughness.
`[0007] For foamed applications, a bloWing agent is added,
`Which can be either a physical bloWing agent such as dis
`solved isobutane, CO2, or a chemical bloWing agent (CBA),
`or both, as is generally knoWn in the art. CBA is generally
`used When density reduction beloW 50 percent are desired.
`When more that 50 percent density reduction is desired,
`physical BloWing agents are preferably injected into the
`extruder, While CBA are still used in smaller amounts as
`bubble nucleators. CBAs require higher temperatures in order
`to activate the CBA and ensure adequate mixing. As the CBA
`is activated, small gaseous bubbles are formed and mixed
`throughout the matrix of the polymer, but the gas produced
`around these bubbles remains in solution in the polymer melt
`as long as the polymer melt pressure remains high. As the
`melt exits the die its pressure drops rapidly alloWing the
`dissolved gas to come out of solution and causing the small
`bubbles to groW. The bubble groWth Will gradually stop as the
`polymer crystallizes as the ?lm cools doWn. If the polymer
`has a viscosity Which is too loW, due to high melt temperatures
`or due to high melt ?oW index of the polymer, or if it does not
`have enough melt strength, the formed cells have a tendency
`to coalesce and eventually burst so the polymer melt Will not
`retain all the bubbles, resulting in poor foaming Thus, viscos
`ity levels and melt strength are important considerations for
`foamed applications. While it Would appear that using loW
`melt index (high molecular Weight) resins Would also be
`helpful in making the melt more viscous, it Was observed that
`such resins generated unWanted shear heating, causing the
`melt temperature to rise too much making foaming di?icult.
`In general, these higher temperatures act to decrease the
`viscosity, and this effect counters the bene?t obtained from
`starting With the more viscous resin.
`[0008] LLDPE resins are knoWn to have poor melt strength
`and this property is further reduced as the Melt index of the
`polymer is increased (that is, the molecular Weight is
`
`Page 4 of 9
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`

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`US 2008/0138593 A1
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`Jun. 12, 2008
`
`reduced). For this reason the use of these resins in non cross
`linked foaming applications has been limited to blends in
`small amounts Where the major component is a high melt
`strength polymer like LoW Density Polyethylene, (LDPE).
`[0009] Accordingly, the methods of increasing MD tear
`strength traditionally used for conventional ?lms, (such as the
`use of loW Melt index LLDPE resins or use of pure or rich
`blends of LLDPE resins in general) are not necessarily appli
`cable for foamed ?lms and thus no foamed sheets having a
`thickness of 1 to 10 mil are knoWn to possess adequate tear
`strengths, particularly MD tear strengths.
`[0010] Surprisingly, it has been found that by using particu
`lar blends of LLDPE and LDPE together With speci?c fabri
`cation conditions, foamed sheets of thin gauge can be made
`With MD tear properties similar to an equivalent gauge non
`foamed sheet of the same composition. In particular, blends
`Which combine a high Ml LLDPE rich fraction With a loW Ml
`branched LDPE minor fraction provide the adequate balance
`of mechanical strength of the polymer base With its melt
`strength, extensibility and stress relaxation, alloWing a thin
`foam ?lm With MD tear strength Which is comparable to non
`foamed counterparts of similar gauge and composition. The
`loWer the density of this high Ml LLDPE resin the better the
`MD tear but at the expense of ?lm modulus. Thus, the foamed
`?lms of the present invention are thin (generally from 1 to 10
`mils thick), and have an MD tear strength of at least 160
`grams for a 3 mil ?lm as measured by ASTM D 1922. The
`foamed ?lms of the present invention are preferably made at
`least in part from blends Which comprise from 10 to 95
`percent by Weight of LLDPE having a relatively high (1.5 to
`6) MI and from 5 to 90 percent of an LDPE resin having a
`relatively loW MI (0.5 to 2.0).
`[0011] The fabrication conditions should be chosen to
`minimize cell size and minimize areas of concentrated stress.
`Such conditions include things such as optimizing die type,
`land length, die gaps, BUR, pressure and temperature pro
`?les, line speed and output.
`[0012] For purposes of the present invention “foamed
`sheets” or “foamed ?lms” should be understood to include a
`single layer in a multilayer structure Where the other layers
`may or may not be foamed sheets of the present invention, or
`a monolayer ?lm, Where the foamed sheet of the present
`invention is the only layer present.
`[0013] The foamed sheet of the present invention is prefer
`ably at least 1 mils (25 microns) thick. While the foamed
`sheets can theoretically be made even thinner than this, tear
`strength rapidly decreases as the size of the bubbles Which
`give the sheet its foamed characteristic, approach or exceed
`the size of the thickness of the sheet itself. The foamed sheets
`of the present invention are preferably no more than 10 mils
`(250 microns) thick, as thicker sheets typically do not need
`the added tear strength achieved by the present invention. If
`additional strength is needed for a particular application hoW
`ever, thicker sheets could be made according to the teaching
`of the present invention. More preferably, the foamed sheets
`are less than or equal to 8 mils (200 microns), still more
`preferably 5 mils (125 microns) or less, and most preferably
`are 2-3 mils (50 to 75 microns) thick.
`[0014] For purposes of this invention the sheet shall be
`considered to be foamed if it exhibits a density reduction of at
`least 10 percent, as determined by an archimedes based
`method, or approximated by the equation densityI?lm vol
`ume/?lm Weight. It should readily be understood that greater
`reductions in density are possible, particularly in thicker
`
`?lms. It should be noted, hoWever that tear strength generally
`drops With greater reductions in density, and so may be a
`limiting factor for a particular ?lm. In general reductions in
`density of betWeen 20 and 50 percent are most preferred for
`the thin ?lms of the present invention. More preferably, the
`?lms exhibit a density reduction of at least 25 percent, and
`most preferably at least 30 percent, With a more preferable
`maximum of 40 percent, and a most preferred reduction of
`density of no more than 35 percent.
`[0015] The foamed sheets of the present invention have
`increased physical properties compared to previous foamed
`sheets of similar thickness. For example the foamed sheets of
`the present invention have a tear strength in the machine
`direction of at least 160 grams for a sheet of 3 mil thickness,
`as measured by ASTM D 1922 Eilmendorf tear type B
`method. Preferably the MD tear strength of this 3 mil foamed
`?lm is at least 250 g, more preferably 360 g and most prefer
`ably above 525 gr, Which is similar to the MD tear strength of
`non foamed ?lms of the same composition. A foamed sheet of
`the present invention having a thickness of approximately 3
`mil (75 micron) also preferably have a tear strength in the CD
`direction of at least 650 gr, more preferably 800 gr and most
`preferably above 1000 gr. At a thickness of 3 mil and above
`(75 micron), it Was observed that the relationship betWeen
`?lm thickness and tear strength Was generally linear. Thus, it
`is preferred that the MD tear strength of the foamed ?lm be
`greater than 50 grams/mil, more preferably greater than 100
`grams/mil, even more preferably greater than 200 grams/mil
`and most preferably greater than 350 grams/mil. Films With a
`thickness less than approximately 3 mils shoW slightly
`reduced MD tear strength, hoWever a ?lm With a thickness of
`less than 3 mil should exhibit an MD tear strength of at least
`25 grams/mil, more preferably greater than 50 grams/mil,
`even more preferably greater than 75 grams/mil and most
`preferably greater than 100 grams/mil.
`[0016] The foamed sheets of the present invention also
`preferably exhibit increased gas transmission properties. Sur
`prisingly, it has been observed that the gas transmission prop
`erties of these ?lms typically increases more than Would be
`expected When considering only the reduction in density.
`Preferably the ?lms of the present invention exhibit a Water
`vapor transmission rate of at least 0.5 g/100 sq-in*day at 3
`mils gauge (normalized 1.5 g*mil/100 sq~in*day as measured
`according to ASTM F1249-90, more preferably greater than
`0.65 g/100 sq-in*day (normalized 1 .95 g*mil/100 sq~in*day).
`Similarly, the foamed sheets of the present invention prefer
`ably exhibit an oxygen vapor transmission of at least 200
`cc/100 sq-in*day (normalized 600 cc*mil/100 in~sq*day as
`measured by ASTM method D3985-81, more preferably
`greater than 270 cc/100 sq-in*day (normalized 877.5 cc*mil/
`100 in~sq*day).
`[0017] The ?lms of the present invention exhibit equivalent
`to loWer blocking When processed in bloWn ?lm equipment,
`as compared With nonfoamed sheets of the same composition
`and gauge. They have a pearlescent appearance and a soft and
`silky textile touch With appeal to various consumer, hygiene
`and packaging markets. Due to its foamed nature, less amount
`of resin is required to provide an equivalent perceived thick
`ness. Or, the same amount of material gives a higher per
`ceived thickness, proportional to its density reduction. Also
`its foamed nature provides perceived sound and temperature
`insulation properties as Well as added cushioning properties
`When compared to non foamed ?lms of the same composi
`tion. The ?lms of the present invention also exhibit static cling
`
`Page 5 of 9
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`

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`US 2008/0138593 A1
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`Jun. 12, 2008
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`reduction and reduced blocking, so could be used in protec
`tive ?lm applications Without the need for antislip or anit
`block additives.
`[0018] Films of the present invention can be easily printed
`With reduced levels or even elimination of corona treatment
`due to its natural surface roughness Which provides and
`enhances mechanical binding to the ink.
`[0019] While not intending to be bound by theory, it is
`hypothesiZed that the increased tear properties and overall
`toughness may be related to the foamed sheets of the present
`invention having a very ?ne cell structure With homoge
`neously dispersed bubbles. The preferably rich LLDPE
`blends of the present invention can produce a very small and
`homogeneous cell structure shoWing betWeen 60 to 100 cells
`per inch in the machine direction, and 90 to 120 cells per inch
`in the transverse direction. The thicker ?lms (8 mils) Will give
`the smaller cells When vieWed in the MD direction (100
`cells/inch), While the thin ?lms (2 mils) give larger cells (60
`cells/inch) as the cells are gradually elongated in the machine
`direction and narroWed in the transverse direction as the ?lm
`is thinned doWn. Accordingly, When vieWed in the transverse
`direction it Will have thinner elongated bubbles Which Will
`shoW a higher cell count (90 cells/inch in a 2 mil ?lm and 120
`cells/inch in an 8 mil ?lm). It is also theorized that the foamed
`sheets of the present invention exhibit loWer crystalline ori
`entation When compared to both LDPE rich blend counter
`parts, and even to some very rich LLDPE blends (>80 per
`cent). The loWer crystalline orientation Would then contribute
`to explain the improved MD tear and toughness seen in the
`unique blends used in the foamed ?lms of the present inven
`tion
`[0020] Another variable in the foamed sheets of the present
`invention is the amount of collapsed cells and/or bubble coa
`lescence observed in the foamed sheets. Larger cells resulting
`from coalescence can cause a Weak spot in the sheet, and thus
`should be avoided. Similarly, collapsed cells may Weaken the
`overall properties of the sheets Without providing any density
`reduction bene?t.
`[0021] The foamed sheets of the present invention can
`advantageously be made from polyole?n blends of LLDPE
`having a relatively high melt index (as compared With LLDPE
`resins normally used in bloWn ?lm applications) and LDPE
`With a fractional MI, The preferred blends of the present
`invention have an LLDPE component With a density range of
`from 0.912 to 0.925 g/cc (as measured byASTM D-792), and
`a melt index (12) of 1.5 to 6 (as measured by ASTM D-1238
`(190o C./2.16 kg). More preferably the LLDPE has an MI in
`the range of 2.0 to 4.5. The LLDPE suitable for use in the
`present invention are generally as described for componentA
`in U. S. patent application 2003/0032731, herein incorporated
`by reference in its entirety. Accordingly they may be homo
`geneous or heterogeneous polymers and can be made accord
`ing to any means knoWn in the art.
`[0022] The LLDPE suitable for use in the present invention
`can be an interpolymer of ethylene With at least one C3-C2O
`alpha-ole?n, as stated in US. 2003/0032731. Preferably the
`LLDPE is a copolymer of ethylene With butene, hexene, or
`octene, With octene being the most preferred. The LLDPE
`may be linear (that is, With no long chain branching) or
`substantially linear. The LLDPE may advantageously be
`made using a gas phase process or a solution process as is
`knoWn in the art. Similarly, the catalyst used to make the
`LLDPE is not limited and includes Ziegler-natta type catalysts
`as Well as metalocenes.
`
`[0023] In general, it has been observed that using LLDPE
`resins With loWer MI was observed to cause more shear heat
`ing making dif?cult to keep melt temperature loW enough for
`good foamability. Additionally loW Ml resins cause excessive
`orientation in the ?nal foamed ?lm, causing a loW MD tear
`properties. On the other hand, it Was also observed that using
`LLDPE With higher Ml led to di?iculties in foaming due to
`loss in melt strength. The use of loWer density LLDPE resins
`contribute to better MD tear properties, but, it reduces the ?lm
`Secant modulus Which can be undesirable in some packaging
`applications. Accordingly, the polymer selection can be
`manipulated to optimiZe the required processability, melt
`strength, melt extensibility and stress relaxation to make a
`microcellular foamed ?lm structure With relaxed and bal
`anced MD/TD tear properties for a particular manufacturing
`system.
`[0024] The LDPE component of the preferred blends for
`use in the present invention have a density range of from
`0.917 to 0.925 g/cc (as measured by ASTM D-792), and a
`melt index (12) of 0.2 to 7.0, more preferably less than 2, and
`most preferably less than 1.0 (as measured by ASTM D-1238
`(190o C./2.16 kg)). Preferably, the MI is less than 3, more
`preferably less than 2, and is greater than 0.5.
`[0025] The LDPE resin used is a branched homopolymer or
`interpolymer made in tubular or autoclave reactors at pres
`sures above 14,500 PSI (100 Mpa) With the use of free radical
`initiators. The LDPE suitable for use in the present invention
`can be selected from the broad class of compounds described
`as component B in US 2003/0032731. Accordingly, the
`LDPE is preferably an ethylene homopolymer but can be an
`interpolymer With one or more alpha or beta ethylenically
`unsaturated comonomers such as acrylic acid, methacrylic
`acid and vinyl acetate. Similarly, the catalyst used to make the
`LDPE is not limited and includes Ziegler-natta type catalysts
`as Well as metalocenes.
`[0026] The LDPE component can also be optimiZed for a
`particular system, folloWing the same general trends as for the
`LLDPE component. Thus, an LDPE With a loWer MI is asso
`ciated With increased melt strength but also causes shear
`heating making di?icult to keep melt temperature loW enough
`for good foamability. Additionally loW Ml resins have been
`associated With excessive orientation in the ?nal foamed ?lm,
`causing a loW MD tear properties. On the other hand, it Was
`also observed that using higher Ml resins led to di?iculties in
`foaming due to loss in melt strength. The use of loWer density
`LDPE resins contribute to better MD tear properties, but, it
`reduces the ?lm Secant modulus Which can be undesirable in
`some packaging applications.
`[0027] Preferably the blend comprises at least 10 percent
`by Weight of the LLDPE more preferably at least 30 percent
`and most preferably 70 percent. The blend ideally comprises
`90 percent or less by Weight of the LLDPE and more prefer
`ably less than 80 percent, although higher amounts may be
`possible. The blend preferably comprises at least 10 percent
`by Weight of the LDPE more preferably at least 18 percent
`and most preferably 30 percent. The blend ideally comprises
`less than 70 percent by Weight of the LDPE and more pref
`erably less than 30 percent. It should be readily understood
`that the blend can be optimiZed, depending upon the particu
`lar system. In general the LLDPE portion contributes more to
`the tear strength properties, Whereas the LDPE portion aids in
`processability and foamability. Thus, for example, if a rela
`tively high Ml LLDPE is used, then processability may not be
`as big of an issue and therefore the LLDPE portion may
`
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`US 2008/0138593 A1
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`comprise a larger proportion of the blend. Similarly, a high
`MI LDPE (for example, up to 6 MI) can be used but may
`require higher loading of it (for example 30 to 70 percent by
`Weight LDPE) in order to achieve a blend With higher melt
`strength properties. The blend also contains a chemical bloW
`ing agent (CBA), Which can be added by any means knoWn in
`the art. The use of CBA and other foaming agents is exem
`pli?ed by the teachings to processes of making ethylenic
`polymer foam structures and processing them in Chapter 9 of
`the “Handbook of polymeric Foams and Technology” entitled
`“Polyole?n Foam”, Written by C. P. Park, edited by D. Klemp
`ner and K. C. Frisch, Hanser Publishers, Munich, Vienna,
`NeW York Barcelona (1991), Which is herein incorporated in
`reference. One preferred method is to add an endothermic
`CBA based on sodium bicarbonate and citric acid in a 20
`percent to 50 percent LDPE masterbatch. The CBA should be
`added such that the amount of active CBA in the blend is at
`least 0.25 percent by Weight, more preferably 0.4 percent and
`most preferably 0.6 percent. The CBA is preferably not added
`in amount such that it exceeds 1 .0 percent more preferably 0. 6
`percent.
`[0028] Minor amounts of other materials may also advan
`tageously be used in the blend used to make the foamed sheets
`of the present invention. These include other polymer to pro
`vide added melt strength, foamability, stiffness like PS, SBR,
`PP, SBS slip additives to provide necessary coe?icient of
`friction (COF) and pigments to provide coloring. PIB type
`additives may be added to provide enhanced cling features to
`the ?lms. Process aids could also be added to help reduce
`shear heating, particularly When using loWer MI blends.
`Other additives such as UV stabiliZers, anti-static or ?re retar
`dants may be necessary to provide required functionality for
`speci?c applications, as is generally knoWn in the art. These
`other materials should not be added in an amount greater than
`2 percent, more preferably 0.5 percent and most preferably
`0.1 percent depending on the additive.
`[0029] The fabrication conditions for making the foamed
`sheets of the present invention also play a role in obtaining
`thin sheets With high tear strength. Typically, a medium shear
`barrier screW is used but it is also possible to use other screW
`designs including tWin screWs, and general purpose polyeth
`ylene, PP and PS screWs. The screW should be able to have
`good mixing capabilities to e?iciently disperse the CBA and
`homogeniZe the blend, be capable of processing LLDPE rich
`bends Without generating excessive shear. It should be
`capable of building and maintaining pressure through the
`extruder to deliver a homogenous melt at high pressures
`(3000 to 6000) PSI to the adapter and die. Pressure through
`out the die should be maintained high up to the die lips Where
`a sudden pressure drop occurs in order to minimize prefoam
`ing prior to the die exit. The type of extrusion die used can be
`a common monolayer spider type die designed for high or loW
`pressure operation. LoW pressure dies, typically used for
`LLDPE ?lm extrusion have demonstrated to provide less
`potential pressure variations, that can lead to premature foam
`ing (prefoaming inside the die). The die gap should be no
`larger than 80 mils (thousands of an inch) (2.0 mm), prefer
`ably no larger than 50 mils (1.3 mm) and most preferably no
`larger than 20 mils (0.5 mm). In general larger die gaps Were
`observed to be related to larger foam bubble structure in the
`foaming process, Which is believed to be caused by prefoam
`ing and bubble coalescense inside the die. Larger die gaps are
`knoWn to cause more unWanted MD orientation. When larger
`foam bubbles are obtained the ?lm does not have the pearl
`
`escent effect and soft touch seen in smaller microcellular cell
`type foamed ?lms Which Were obtained With narroWer gaps.
`[003 0] The Land Length of the die (the length of the parallel
`section of the die lips) has an important effect in assuring a
`fast pressure drop at the die lips, With minimum orientation of
`the molecules and loW shear heating minimiZing unWanted
`prefoaming inside the die. Ratios of the land length/die gap
`should be beloW 25, more preferably beloW IS and most
`preferably beloW 12. These smaller ratios are preferred in
`order to obtain small microcellular foam responsible for the
`pearlescent aesthetics.
`[0031] The extruder should use a reverse temperature pro
`?le With a peak temperature of 450° F., in order to fully
`activate the CBA. There is also ideally a gradual decent to a
`die lip temperature of 340° F. The process should have a high
`RPM (60 to 80 percent of the maximum RPM), for example
`90 to 110 RPM for a 21/2 in extruder, With high throughput
`(loW residence times), for example 6-10 lbs/hr/rpm. High
`throughput is equivalent to loW residence time) and a fast
`pressure drop (5000+PSI at the screen pack doWn to 1200 psi
`(or higher) at the die for as feW seconds before the die gap and
`doWn to atmospheric pressure at the die exit When foam
`groWth takes place. Ideal pressures in the extruder can vary
`from 3000 to 6500 psi, While pressures at the die are ideally at
`or above 800 psi. If the pressure at the die drops beloW 600 to
`700 PSI, prefoaming inside the die is likely to result, leading
`to bigger and feWer bubbles and poor aesthetics. Having
`pressure above 5000 psi at the screen pack helps to maintain
`a resultant pressure at the die above 1200 psi after the initial
`pressure drop, Which helps ensure that the polymer reaches
`the die lips With minimal amounts of foaming occurring until
`the die exit.
`[0032] A high BUR Was also seen to be bene?cial for form
`ing the thin foamed sheets of the present invention. It is
`preferred that the ratio be from 2.2 to 4.0 BUR, more prefer
`ably from 2:5 to 3.5: 1. BURs above this range tended to cause
`problems in forming a stable bubble Whereas BURs beloW
`this range tended to be associated With a ?lm having very
`unbalanced properties, particularly very loW MD tear values.
`The use of internal bubble cooling (IBC) can provide addi
`tional cooling and help stability of foaming process.
`[0033] It should be readily appreciated by one skilled in the
`art, that the blend components and fabrication conditions can
`be chosen to optimiZe the chance of successfully making a
`thin foamed sheet of the present invention.
`[0034] The folloWing examples are illustrative of the inven
`tion, but are not intended to limit the scope of the invention in
`any Way.
`
`EXAMPLES
`
`[0035] Thin sheets Were formed from the LDPE and
`LLDPE resins indicated in Table 1. Resin A Was LDPE With
`a Melt Index (MI) (at 190° C./2.16 kg) of2.3 and a density of
`0.920. Resin B Was LDPE With an MI of 0.47 gr/10 min and
`a density of0.920. Resin C Was LLDPE With an MI of0.5 and
`a density if 0.920. Resin D Was LLDPE With an MI of 1.0 and
`a density of0.920. Resin E Was LLDPE With an MI of2.3 and
`a density of 0.917. Resin F Was an ULDPE With MI of4 and
`density of 0.904 gr/ cc. The Chemical BloWing Agent or CBA
`used Was SAFOAM FPE-50 Which contains 50 percent of
`active ingredient of encapsulated sodium salts of carbonic
`and polycarboxylic acids, in a polyethylene carrier. 3 mil
`gauge ?lms Were produced using a 2 .5 inch extruder equipped
`With an 8 inch loW pressure die and medium shear barrier
`
`Page 7 of 9
`
`

`
`US 2008/0138593 A1
`
`Jun. 12, 2008
`
`screW.A 40 mil die lip With 1/2 inchland length Was used. The
`line Was run at 220 lbs~hr rate. MD tear strength Was then
`measured according to ASTM D 1922 Elmendorf type B
`method. Puncture Propogation