2la~ 0 ;- W~92/1201~ ~CT/US92/00221 Heat sealable mult11ayer polyolef1n~c f11m structures.
,, This invention relates to oriented multilayer.
heat sealable thermoplastic film structures. More particularly, the invention relates to multilayer film q structures which retain heat sealability even after 5 irradiation.
Many polymers have been developed which can withstand exposure to irradiation, often incorporating .! lo specific additives or antioxidants which prevent polymer degradation. Films and multilayer films made from these and other polymers are sometimes exposed to irradiation i during processing or u~e. For example, irradiatian is used in some lnstances to de~troy micro-organis,ms in the 15 wrapped product (e.g., in food ~terilization) and to , .
also prepare at least one component of the film for thermal shrinkage at a later time.
Multilayer film packaging has many requirements 20 for both enduring the irradiation exposure and preparing . part of the structure for ~ubsequent shrlnkage.
Typically, the package ls a multllayer structure, with' the different layers performlng different runctions.
25 Usually the layers ar~ made from thermopla~tLc polymers.
,~ ' ' , ,, ,............................................................... .
. , wo 92/12010 2 ~ ~ ~ 3 ~ ~ -2- PCrtUS92/0~221~
For example, one layer may be a barrler layer (e g., ethylene vinyl alcohol (EVOH)), the heat seal layer may be a polyethylene (e.g., linear low density polyethylene < (LLDPE)), while still another type of polyethylene (e.g., high density polyekhylene (~IDPE)) mlght be used 5 as the structural or core layer.
Irradiation of multilayer film structures can be done by exposure to Cobalt 60 (gamma irradiation) or by electron beam (beta). When the entire multilayer film structure is irradiated, the heat sealability of the heat seal layer often decreases (i.e., the intiation temperature of the heat sealing increases), especially , when the heat seal layer is polyethylene, while the ; 15 physical properties (e.g., strength) of the structural layer increase. Reduction in heat sealability causes lower heat seal strengths and narrower heat sealing temperature ranges. Exposure to irradiation, especially beta irradiation, and subsequent exposure to heat can 20 also cause the core layer and the heat seal layer to shrink, especially when the multilayer structure has been oriented (e.g., film). Controlled shrinkage of the core layer is desirable, but a narrower heat sealing 25 temperature range of the heat seal layer $s not desirable.
Irradiation of the heat seal layer also results in higher heat seal initiatLon temperatures a-~ well as 30 narrower heat seal temperature range3. The narrow heat seal range of the heat seal layer can cause the wrapped product to become exposed to the external environment and contamination, khus negating the sterilizing efrects of irradiatlon.
WO92/12010 PCI/US~2/00~21 USP 4,927,708 (Herran et al.) dl~closes an improvement in multilayer thermoplastic fllm havlng at least 5 layers wherein a core layer comprl3e~ a very low density polyethylene, intermediate layers adjacent each side of the core layer comprising a linear low denslty polyethylene and two outer layers each bonded to the respective intermediate layer comprising a very low density polyethylene. The very low density polyethylene can have a density between 0.890 and 0.915 grams/cublc centimeter (g/cm3). There i5 no disclosure as to the molecular weight distribution, nor i3 there any reference to the short chain branching distribution of any of the layers.
15USP 4,798,081 (Hazlitt et al.) discloses a method and apparatus for analyzing a solution of a crystalline or semi-crystalline polymer sample (e.g., polyethylene). The analysis includes temperature rising elution fractionation (TREF) which rneasures the degree of branching of select fractions of the polymer sample.
USP 4,755,419 (Shah) dlscloses a 7-layer coextruded oriented film comprlsing a core layer o~
ethylene-vinyl alcohol copolymer, two intermediate layers each comprising a polyamide, two outer layers each comprising a blend oP polymeric materials and two --layers each comprising an adhesive polymeric material.
The blend of polymeric materials are taken from the group consisting of (i) a blend of linear low density polyethylene, a linear medlum denslty polyethylene and an ethylene vinyl acetate oopolymer and (11) a blend of an ethylene propylene oopolymer end a polypropylene.
~'' ' ' - .
WO 92/12010 PCr/US92/0022Ir~
USP 4,724.185 (Shah) discloses a coextruded multiple layer oriented film. The core layer compri3es a cross-linked blend of ethylene vinyl alcohol copolymer and polyamide. Two cross-linked interior layer~ each comprising adhesive re3ins and two aros~llnked outer layers each comprising a 3 component blend of linear 10W
density polyethylene, linear medium density polyethylene and ethylene-vinyl acetate, are each included in the multiple layer film.
USP ~,668,752 (Tominari et al.) disclose~
ethylene/C4-C20 alpha-olefin copolymers which have new characteristics in regards to composition distribution.
degree of branching. randomness and crystallinity. The ethylene copolymers have various properties, including a melt flow rate of from 0.01 to 200 gtlO minutes, a molecular weight distribution measured as Mw/Mn of 2.5 c Mw/Mn - 10, a density of from 0.850 to 0.930 g/cm3, the amount of components having a degree of branching of not more than 2/1000 carbons of not more than 10 percent by weight based on the ethylene copolymer and the amount of components having a degree of branching of at least 30/1000 carbons of not more than 70 percent by weight based on the eth~lene copolymer. The disclosure alleges that the ethylene copolymers made using this invention have excellent mechanical properties, optical properties, blocking resistance, heat resistance and low temperature heat sealability in a well balanced combination. Tominari et al. do not specif~ how the ethylene/C4-C20 alpha-olefin copolymers perform when exposed to irradlatlon, much less how they perform when in a multilayer structure that is exposed to irradLatLon.
WO92/12010 ~ & ~13 ~ PCT/US92/00221 USP 4,640.856 (Fergu~on et al.) discloses multi-layer thermoplastic shrink film having improved shrink, toughness and barrier properties achieved by a substrate layer of very low density polyethylene and a gas barrier layer of vinylidene chlorlde copolymer or ethylene-vinyl alcohol. The very low density polyethylene can have a density of less than 0.910 g/cm3 and as low as 0.860 g/cm3Or even lower. there is no disclosure as to the molecular weight distribution. nor is there any reference to the short chain branching distribution of the substrate layer.
USP 4,352,849 (Mueller) discloses multilayer heat shrinkable packaging film comprising a core layer consisting essentially of an ethylene-propylene copolymer blended with an ethylene-vinyl acetate copolymer and a skin or surface layer comprising:an ethylene-propylene copolymer.
USP 4,064,296 (Bornstein et al.) discloses a multi-layer packaging film having low oxygen permeability aomprising two polymeric layers, at least one of which is cross-linked, and a layer between the polymeric layer~ comprising a hydrolyzed ethylene vinyl acetate copolymer. The two polymeric layer~ can compri~e polyethylene, although the type of polyethylene is not specified.
USP 4,~20,557 (Warren) discloses a packaglng film comprising at least one layer of a linear copolymer ' -of ethylene and an alpha-olefin with a density o~ about 0.935 g/cm3 or les~ and a low Ilo/I2 ratio. The film allegedly ha~ better abu~e re~lstance as compared with a film made with a linear ethylene/alpha-ole~ln copolymer .
1, , .... . . . . .
,.......................... . .
2 ~ 3 ~ ~
WO92/12010 PCr/U592/0022li-having a comparatively higher I1o/I2 value. Warren is directed to use of polyethylene which has a narrow molecular wei~ht distribution in order to improve abuse resistance of multilayer structure-~. Warren specifically teaches away from the use of polyethylene having a broad molecular weight distribution (i.e..
higher I10/I2 values) in multilayer film structures.
In spite of these advances, a better balance of heat seal and shrinkage properties of the overall structure is still needed before and after irradiation - so that the overall integrity of the package is not lost a~ter irradiation.
A multilayer heat-sealable thermoplastic film structure has now been discovered that has improved heat seal performance after irradiation. The novel multilayer ~ilm structure comprises at least one heat sealable outer layer (A) and at least one core layer (B) Layer (A) of the multilayer thermoplastic film structure comprises a first linear polyethylene characterized as having:
(a) a density from 0.88 grams/cublc centimeter (g/cm3) to 0.92 g/cm3, (b) a melt index, measured as I2, from 2 grams/10 minutes to 20 grams/10 minutes.
; (c) a molecular weight distrlbutlon. measured as I1o/I2~ from 5 to 8. and whereln (d) 15 percent by weight or less of said llnear polyethylene ha~ a degree of branching le~s than or ; equal to 2 methyls/1000 carbons, and . . .
~ ' .
, ' ,: i WO9~ 010 ~ ~ a ~ Pcr~us92~0022l (e) 25 percent by weight or less of said linear polyethylene ha~ a degree of branching equal to or greater than 25 methyls/1000 carbons.
Layer (B) comprises a second llnear - polyethylene characterized as having: I
(a) a density from 0.88 g/cm3, to 0.94 g/cm~. !
(b) a melt index, measured as I2, from 0.05 grams/10 minutes to 5 gram/10 minutes, (c) a molecular weight distribution, measured as I10/I2, from 8 to 30, Figure 1 shows the preparative scale temperature rising elution fractionation apparatus used to determine the branching distribution in the polymer fractions.
Figure 2 plots heat seal strength (measured in Newtons/inch) versus heat seal temperature (in F) of three ultra low density polyethylene (ULDPE) resins made into film.
Figure 3 plots cross-link clensity (measured in percent gel) versus radiation dosage (in megarads) of two ULDPE resins made into film.
Figure 4 plots heat seal st,rength (measured ln Newtons/inch) versus heat seal temperature ~ln F) of one ULDPE resin made into film after dlfferent levels of - exposure to irradiation.
Figure 5 plots heat seal strength (measured in Newtons/inch) versus heat ~eal temperature (in F) of one ULDPE resin made into film after different levels of expo~ure to irradiation, Figure 6 plots cross-link dens~ty ~measured in percent gel) versus radlation do~age (ln megarads) of one LLDPE resln made lnto fllm, ~'''' . ''' ' , ' ~
,i '': "
WO9~/12010 PC~/US92/00221 Figure 7 depicts one multilayer film structure of the present invention.
The multilayer 3tructures of the pre~ent invention comprise at least one heat-sealable outer layer and at least one core layer. The multilayer structure can have many layers, but the heat sealable layer of the present invention must be on at least one outer surface of the structure to take advantage of the improvement in heat seal characteristics. Typical multilayer stru¢tures can be made using a coextruded blown or cast film process. Figure 7 shows a multllayer structure of the present invention. The structures of the film product can generally be described as an A/B/D/X multilayer structure wherein layer A (18) is the heat sealable outer layer, layer 8 (19) is the core layer and layers D (20) and/or X (21) are a tie layer(s) and/or a barrier layer(s). The core layer (19) can be adjacent to the heat seal layer, but does not have to be. The multilayer film structure is comprised of from 2 to 20 layers, preferably from 2 to 7 layers.
The multilayer film structure can be irradiated at any point during the process. For example, the heat-sealable outer layer and the core layer of the pre~ent invention can be coextruded, irradiated and ~ubsequently laminated and oriented (i.e., stretched) to make the final multilayer film ~tructure, so long as the heat-sealable outer layer is on at lea~t one outer surface ofthe structure. Converaely, the entire multllayer structure can be formed and oriented, and then ~ubsequently irradlated. The order of the ateps of orientatlon and lrradiatlon ls not crltLcal.
Irradlatlon is pre~erably accompllahed by uslng an ;'''''''''' ''''' '''' , ~' , W092/12010 2 :L ~ 6 S Pcr/us92/oo221 electron beam (beta) irradiation device at a dosage level of from 0.5 meKarad (Mrad) to 20 Mrad.
Thickness of the multilayer f~lm structures can vary from 0.1 mil to 50 mils (.0025 mm to 1.3 mm), preferably from 0.5 mils to 10 mils. (.013 mm to .25 mm). The tie layer(s) and/or the barrier layer(s) are made from conventional thermoplastic polymers known to be useful for this purpose, especially polyolefinic Plymers Thermoplastic polymers useful as the tie layer(s) include ethylene acrylic acid interpolymers (e.g., U.S. Patent 4,599, 392), ethylene methacrylic acid interpolymers (e.g.. U.S. Patent 4,248,990 and U.S.
Patent 4, 252, 924), succinic acid or succinic anhydride grafted polyolefins and blends with ungrafted polyolefins (e.g., U.S. Patents 4, 684,576. 4.741,970 and 4, 394,485), vinyl acetate interpolymers. Thermoplastic polymers useful for the barrier layer(s) include EVOH, polyvinyl chloride, polyvinylidene chloride, and nylon.
The heat-~ealable outer layer and the core 25 layer of the present invention are fabri¢ated from linear polyethylene. Manufacture of linear polyethylene is disclosed. e.g., in U.S. iatent 4,076,698 and involves coordination catalysts of the "Ziegler" type or "Phillips" type and includes variations of the Ziegler type, such as the Natta type. These catalysts may be used at very high pressures, but may also (and generally are) used at very low or intermediate pressures. The products made by these coordination catalysts are generally known as "linear" polymers because of the substantial absence of branched chains of polymerized , ' , . . . . .
,~ , , .
2la~3~s WO92/1~010 PCT/US92/002~
monomer units pendant from the main polymer "backbone."
It is these linear polymers to which the present invention pertains. Linear low density polyethylene (LLDPE) and ultra low density polyethylene (ULDPE) typically have a density between about 0.88 g/cm3 and about 0.94 g/cm3. The density of the polyethylene is lowered by copolymerizing ethylene with minor amounts of an alpha, beta-ethylenically unsaturated alkene(s) having from 3 to 20 carbons per alkene molecule (e.g., l-propene, l-butene, l-pentene, 4-methyl-l-pentene, l~
hexene, l-octene, l,9-decadiene and l,7-octadiene), preferably 4 to 8 carbon atoms (e.g., l-butene, l-hexene and l-octene) and most preferably 8 carbons per alkene molecule (i.e., l-octene). The amount of the alkene comonomer is generally sufficient to cause the density of the linear low density polymer to be substantially in the same density range as LDPE, due to the alkyl side chains on the polymer molecule, yet the polymer remains in the "linear" classification; they are conveniently referred to as "linear low density-polyethylene." These polymers retain much of the strength, crystallinity, and toughness normally ound in HDPE homopolymers of ethylene, but the higher alkene comonomers impart high "cling" and "block" characteristics to extrusion or cast films and the high "slip" characteristic inherently found in HDPE is diminished.
The use of coordination-type catalysts or copolymerizing ethylene with higher alkenes to make LLDPE and ULDPE copolymers having densities between about 0.88 g/cm3 and about O.g4 g/cm3 is disclosed variously in, e.g., U.5. 2,699,457; U.S. 2,846,425; U.S.
2,~62,917; U.S. 2,905,6~5; U.S. 3,058,963; U.S.
4,076,698; and U~S. 4,668,752. The den~ity of the ~J _t ~ ~ ,7~J
~-WO92/12010 PCIYUS92/~022l _ 1 1 _ linear polyethylene useful in the heat sealable outer layer (A) in the present invention is from 0.88 g/cm~ to 0.92 g/cm3, preferably from 0.89 g/cm3 to 0.915 g/cm3.
The density of the linear polyethylene u~eful in the core layer (B) in the present invention is from 0 88 g/cm3to 0.94 g/cm3, preferably from 0.9 g/cm3 to 0~93 g/cm3.
The molecular weight of the LLDPE useful in the present invention is indioated and measured by melt index according to ASTM D-1238, Condition (E) (i.e., 190C/2.16 kilograms); also known as I2. The I2 of the linear polyethylene used in the heat sealable outer layer (A) can be as low as 0.5 grams/10 minutes, but is preferably from 2 grams/10 minutes (g/10 minutes) to 20 g/10 minutes, more preferably from 3 g/10 minutes to 10 g/1C minutes. The I2 of the linear polyethylene used in the core layer (B) can be from 0.05 g/10 minutes to 5 g/10 minutes, preferably from 0.2 g/10 minutes to 1 g/10 minutes.
The molecular weight distribution is indicated and measured by I1o/I2 accordlng to ASTM D-1238, Conditions (N) (190C/10 kLlograms) and (E), respectively. The I1o/I2 of the heat sealable outer layer (A) can be from 5 to 8, preferably from 6.5 to 8.
The I1o/I2 of the core layer (B) can be from 8 to 30, , preferably from 8 to 15. Molecular weight distribution can be achieved by varying catalytic conditlons, reactor conditions or feed percentages. Blends of polyethylenes can also be used. Such blends can be prepared by blending separate and dlscrete polyethylene polymers, or polymerizing the polyethylene polymer in-~ltu ln multiple reactors, (e.g. the technique di~cloqed ln U.S.
Patent 3,914,342). It i~ important that the llnear "
,, ' '''',' ' :
W092/12010 PCr/US92/On221~'-polyethylene(s) be well mixed when u~ing discrete polymer blends.
Additives, (e.g., anti-oxidant3, plgments, hydrophobic and/or hydrophillc agents. etc.), may be incorporated into the lin0ar polyethylene to the extent that they do not interfere with the heat sealability of the outer layer and the shrinkability of the core layer after orientation and exposure to irradiation.
Short chain branching distributlon (SCBD) of the polymer fractions of the polyethylene polymers ; useful in the heat seal layer and the core layer Ls measured according to the temperature rising elution fractionation (abbreviated herein as "TREF") ~echnique described herein and in U.S. Patent 4,798,081.
Experimental Procedure for Determinin~ Short Chai~
I. TREF Device Design A diagram of the TREF apparatus is shown in Figure 1~ The column (1) is 4 inche~ (10 cm) in - diameter and 29 inches (74 cm) Ln length, a size easily capable of fraotionating up to 20 grams of sample. The column is packed with stainless steel~shot (2) and heated by a circulating oil bath (3). The entire apparatus is enclosed within a hood (not shown) to contain any solvent or heating oil vapors.
A. Column Construction The column is constructed-of 4 Lnch (10 cm) schedule 40 ~teel pipe. Leg~ of one inch ~2.5 cm) angle iron were welded on the pipe to keep lt in a vertlcal ~092/12010 2:~ ~?~ pcr/us92/no~2l position. The end caps are welded on the pipe to prevent any solvent leakage into the oil system. A one inch (2.5 cm) hole is left in each end to allow for packing of the column and to accomodate a 1 inch MPT
(2.5 cm) x 1/4 inch (0.6 cm) tubing connectlon.
B. Column Packing The column is tightly packed with 0.028 inch (.07 cm) diameter stainless steel shot. The interstitial volume of the column is approximately 1700 milliliters. Stainless steel shot effectively prevents channeling and provides good heat transfer through the column.
C. Heating System The column temperature is regulated by a:Haake N3B circulating bath controlled by a Haake PG-20 temperature programmer (4). The oil used is Primol 355 mineral oil or a light silicon oil. The 10 gallon (38 l) insulated bath (5) containing the column is galvanized steel. The oil circulator injects oil at the ` bottom of this bath with the return from the top. An air driven motor (6) is used for agLtation of the large oil bath- , D. Solvent System The solvent used ls 1, 2, 4-trichlorobenzene (TCB) (12). It is pumped from a reservoir (7) lnto the bottom of the column through a preheating coil (8) located in the large bath. The solvent iq pumped at a flow rate of approxlmately 40 ml/mlnute using a Fluid Metering, Inc. lab pump (9) designated ~P-C-150 wlth a 2-SSY ~3/8 inch or 0.95 cm) stainless steel pump head .... .. . . . . ....
W092/1~010 ~l/US'l2/0022l module. This pump allows control of flow rates from 1 to 100 milliliters/minute.
E. Column Loading System Polymer samples (13) are loaded on the column utilizing a gravity flow method. The sample is first dissolved in 1700 ml of TCB in a heated 5 liter flask (10). The 130C column is then loaded with the solution, also at 130C. One hundred milliliters of clean solvent is then heated and added to flush all transfer lines. These lines are heated with rheostat controlled heat tape (not shown).
F. Fraction Collection The column eluent (14) is collected in one gallon metal cans (11). Acetone is then added to:the cans to precipitate the polymer. Fraction work-up will be discussed in a later section.
II. Fractionation Procedure and Column Operation ' A. Loading the Column The polymer sample (about20 grams) iq dissolved l 25 in 1700 ml of TCB in a heated 5 liter flask.
,~ Approximately 3 to 4 hours is required to carry out this step. The column, the flask, and all transfer lines must be above 120C to insure that no polymer is precipitated during the loading step. After the polymer solution is deposited on the column, a wash of fresh solvent is used to eliminate all polymer from the transfer lines. The amount of wash ls dependent on the volume o~ the transfer lines (about 100 ml for this system). After completing the~e step~, the entire 2:~i;13~rl WO92/12010 ~JCr/US'J2/0~2~1 system must be brought back down to room temperature.
with the rate of cooling being the most critical step in the fractionation procedure. The rates for thl~ system are between 1C and 2C/hr. A temperature recor~er ~15) equipped with a thermocouple (16) in the recirculatlng oil and a thermocouple (17) in the packed column is used to monitor the temperature changes. During the cooling step the polymer selectively crystallizes allowing for efficient fractionation during the elution step. There is no solvent flow through the column during the cool-down step.
B. Column Elution Elution of the polymer from the column is started at room temperature with a predetermined temperature rise rate. Solvent flows continuously during this step and fractions are collected over the desired change in temperature (5C for this work). The temperature rise rate used here is 6C/hr. The temperature rise can be achieved in two ways, stepwise or continuous. Solvent flow rates (40 ml/min) are chosen to aehieve one interstitial column volume over the desired change in temperature. These rates result in a fraction collection every 50 minutes.
-~ Approximately 15 fractions are collected for each sample.
C. Fraction Work-Up The fractions are mixed about 1:1 with acetone and allowed to ~it for at lea~t one day to precipitate the polymer. The polymer-TCB-acetone mixture is then added to a large ~eparatory funnel, and allowed to sit for several minutes untLl the polymer rlses to the top.
2~ 0 WO92/12~10 PCr/US92/nO221,-The bulk of the solvent is then drained and more acetone added to wa~h the polymer. The acetone-polymer mixture is then vacuum filtered, and the resulting polymer sample dried in a vacuum oven at 80-90C followed by weighing. These samples can then be directly u~ed ~or gel permeation chromatography, differential scanning calorimetry, and infrared analysis.
D. Branching Content Determination Branching content (i.e., degree of branching) is calculated from CH3/1000 C (methyls/1000 carbons) determination according to ASTM method ~2238-68. A
Beckman 4260 infrared spectrophotometer is employed.
using films of approximately 0.15 mm thickness. A
correction for chain end methyl groups is necessary for accurately determining weight percent comonomer (e.g., l-octene) incorporation. The correction is done according to the following equation:
Corrected CH3/1000C = Uncorrected CH3/1000C +~inyl _ 2800 1000 Mn Comonomer incorporation can be determined from the following equation:
Wt.% octene - (Molecuiar Weiqht of 1-octene) (Corrected CH~/1000C) (100) 14000 ~ 84 (corrected CH3/1 000C) E. Molecular Welght Determinatlon 2~
WO92/12010 PC'r/1).~9~lnOZ21 Number average molecular weight (Mn) ls measured by using a Waters Model 150C Gel Permeation Chromatograph. The measurements are made by dissol~/ing polymer samples in hot, filtered, l, 2, 11 - trichlorobenzene (TCB). The GPC (Ge:L Permeatlon Chromatography) runs are made at 140C in TCB. A flow rate of l.0 ml/min is used and the columns used are 3 1 Polymer Laboratories 10 micron linear columns. Each column diameter is 7.5 mm and the column length is 30 cm. Column performance is typically around 307000 plates/meter a.s determined using 0.02 grams eico~ane in 50 milliliters of TCB. Columns are dispo~ed of if the . plate count is below 20,000 plates per meter. Column performance is also monitored using the multiplied -I 15 product of the spreading factor "a" and the slope of the ~ calibration curve "D". This value is typically around - 0.081. Columns with values above 0.09 for the multiplied factor ''Dv~'' are not employed. The antioxidant butylated hydroxytoluene is added to the TCB
at a cGncentration of 250 parts per million. The system is calibrated using narrow molecular weight polystyrene standards. The following formula is used to transform polystyrene molecular weights to polyethylene molecular weights:
Mw of polyethylene = (0.4316) (Mw of polystyrene) . , The polyethylene samples are prepared at a concentratlon of 0.25 gram~ of polyethylene in 50 millillters of TCB.
The volume injected l~ lO0 microliters.
WO92/12010 PCr/US~2/00 F. Infrared Determination of Vinyl Content A Perkin-Elmer Infrared (IR) Model 760 i3 uged to measure vinyl groups in the polymer chaln3. The polymer is compression molded into a thin film and measured for thickness (target is approximately 0.1-0.3 mm thick). The IR absorbancy spectrum is measured for each film and vinyl group content is calculated according to the following equation:
Vinyl Groups/ = IR Absorbance at .
1000 Carbons in the polymer chain (t) (k) where: t=thickness k=0.970 and A_909 cm~1.
The vinyl groups/1000 carbons value obtained is inserted into the equation for determining corrected CH3/1000 carbons for each polymer fraction as described earlier in Section D in this disclosure.
The first linear polyethylene suitable for use as the heat seal layer (A) in the pr~s3ent inventlon has a degree of branching le~s than or equal to 2 methyls/1000 carbons in 15 percent by weight or less.
more preferably 10 percent by weight or less, of the linear polyethylene and a degree of Dranching equal to or greater than 25 methyls/1000 carbons in 25 percent by weight or less, more preferably 20 percent by weight or less, of the first linear polyethylene.
;WO92/12010 2~ ~t~ Cr/USn/0022l _19_ Example 1 This example demonstrates heat ~eal performance of three ULDPE Resins with dlfferent short chaLn branching distributions (SCBD).
ATTANE* 4002 ULDPE (labeled as Resin A), an ethylene/octene-1 copolymer made by The Dow Chemical Company, having I2 = 3.3 gram~/10 minutes. Ilo/I2 - 7-7 and density of about 0.912 g/cm3 and ATTANE~ 4001 ULDPE
(labeled as Resin B), also an ethylene/octene-1 copolymer macle by The Dow Chemical Company, having I2 ~
1.0 gram/10 minute, I10/I2 = 8.2 and density of about 0.912 g/cm3 and another ULDPE resin (labeled as Resin C), also an ethylene/octene-1 copolymer, having I2 - 3-3 grams/10 minutes, I1o/I2 = 7.7 and density of about i 0.912 g/cm3 were made into single layer cast film and evaluated for heat seal performance. Each film sample i~ was heat sealed and then tested for heat seal strength.
Resins A and B had similar short chain branching distributions but differed in I2 and I1o/I2.
Resin C had a "narrower" short chaln branchlng distribution as compared to Resins A and B, as measured using TREF, since about 8.5 percent (total weight basis) of Resin C had a degree of branching of less than or equal to 2 methyls /1000 carbons and about 18.5 percent (total weight basis) of Resin C had a degree of ;~ branching of greater than or equal to 25 methyls /1000 carbons. The physical propertie~ of the resins used in all of the experiments described herein (Resins A, B, C
and D) are ~ummarized in Table 1.
...... .. . .. ... .... .. . . . . . . .
wo 92"2~l0 2 ~ ~ t~ PC~/US92/(J022~
--~0--Resin Physical Propertie3 . _ ..
Weiyht Weight . Percent Percent polymer polymer I 2 fraction fraction RESIN 10 minutés) I10/I2 (Dg/cSIt3) degreeof havlnga branchingof branchinqof ~ 2 methyls :~ 25 met~yls carbons carbons _ _ . _ _ .
3.3__ 0.912 16.5 _ 7 . _ 1.08.2 0.912 16.5 27 :: C 3.37.7 0.912 8.5 18.5 D 0.55 12 0.923 NM NM
NM - Nol , Measure ~d -- Samples of single layer 1 mil thick films were fabricated ~rom Resins A, B and C using a cast film line equipped with a MPM extruder with a 2 inch (5 cm) screw (L/D=24:1) a 12 inch (30 cm) cast film die and a MPM
cast film take-up system with a 18 inch (46 cm) chrome plate chill roll and an air knife to cool the polymer mel.t. The fabrication conditions of the cast film are listed in Table 2.
3o ' ~ WO92/12010 2 ~i~13~ Pcr/us92/0022l Fabrication Conditions -for Samples ~ ... ,... .
ExtruderZone 1 (C) 200 . , Extruder Zone 2 (C) 240 .
Extruder Zone 3 (C) 240 . __ _ . _ , , . _ I
Die Temperature (C) Z40 MeltTemp. (C) 2a,0 I
~, , _ _ ~
Chill Roll (C) 12 l . ~
~r~e r (rDm) 80 Film Çauge (rnils) 1 (.025 mm) :
... - . . . . .
E~ udel Back Pro~ure ~ps ) l8s0 ( 12.7 UPa) Line Speed (feeVminute)60 (18 m/min) _ Through put (Ibsihrj40 ~18~kg/hr) Die Width (inches) 12 (30 cm) Film Width (inches) 8 (20 cm) Die Gap (mils) 8 (0.2 mm) The cast film samples were compared for heat seal performance and heat qeal strength using an Instron peel test. Film samples for heat ~eal testlng were conditioned a~ described in ASTM ~ 171-82 (i.e., 73.4 ~/- 3.6 F (21-25C), 50~/- 5 percen~ relative ., , ...
.. . .. .
, wo 92/12010 21~ 6 ~ Pcr/us~2/~
~, humidity). The heat seal tests were per~ormed utilizing a heat sealer. A variety of equipment exists for this function, such as a Sentinel Heat Sealer, a Theller Precision Heat Sealer and a Paok~orsk Hot Tack Tester ! tmade by Top Wave). The conditions for sealing were typically 0.5 seoond dwell time, 40 psi (275 kPa) bar pressure. Both bars were heated. One inch (2.5 cm) .J , wide by six inch (15 cm) long strips were used for testing. After sealing, the samples were condLtioned at i,f 10 the ASTM conditions descrlbed above for about 24 haurs.
The heat seal tests were made using an Instron ; tensiometer at a constant crosshead speed of about 2 ~ inches/minute (5 cm/minute) in a 90 "T-peel" mode.
- ~ Three to five heat seal tests were made and tested. The 3 15 results were averaged and are illustrated in Figure 2 and the data are summarized in Table 3.
Resin C had a "narrower" SCBD and exhibited an improved heat seal performance (i.e., lower heat seal 20 initiation temperature and higher se!al strength at below 210F) as compared to Resin A and Resin B. Resin C is thus an example of this invention and can be used as a heat seal layer in a multilayer fil~1 structure.
~ WO9~/12010 ~ ; s35~ pcr/us92/oo22l Heat Seal Strength of Resins A, B, AND C
Heat Sea]. Strength Heat Seal Temperature (Newton~/inch) (F) Resin AResin B Resin C
... ,. . ,, ~ .. _ ~
, 1~0 O O O
~ 'C I . ..
(~8~- ) ~ (3.5 N/cm) 2û0 8 5 21 (93C) (3.1 N/cm) (2 N/cm) (8.3 N/cm) ~_~___ ~ ___ (99 C ) ( 12.2 N/cm)~1 1.8 N/cm) ( 13.4 N/cm) (104C ) .(12.6 N/cm)(13.4 N/cm) (13 N/cm) . ...... _ . __ ( 1 10 C ) ( 1 1.8 N/cm)( 12 .6 N/cm) ( 12 .2 N/cm) ~40 30 32 29 (I ~li 'Cl (1 1.8 N/cm)(12.5 N/cm) (11.4 N/cm) (121C) (11.8 N/cm) (12 6 N/cm) (11.4 N/cm) ~ _ . , Example 2 This example demonstrates heat seal performance of electron beam irradiated ULDPE.
Résin B and Resin C were evaluated for heat ~eal performance and cross-link density (mea~ured by percent gel) prior to and after electron beam irradiation. An Energy Science Inc. Model EC 200 electron beam radlation machine wa~ used for the radiation treatment. The crossllnk denc~ity (percent gel in xylene, measured by AS~M method D-2765) indicates the resins' propensity for heat ~eal performance. The WO92/12010~ 3 ~ Q Pcr/us92too221, cross-link densities of the reslns before and after various doses of radiation treatment are illu-trated in Figure 3 and the data are summarized in Table 4. As the data show, resin C can be exposed to about 5 megarads (Mrad) and still maintain a cross-link density of less than about 10 percent gel and thus is an exampls of this invention. Resin B reached a cross-link density of about 10 percent gel in less than about 3 Mrad exposure and is not an example of this invention for use in the heat seal layer.
Crosslink Density of Irradiated ULDPE
(Resin B and Resin C) __, _ _ ..... __ Crosslink Density ( Percent Gel) Radiation Dose (Mrad) Res1n B Resln C
~ O : ' - ~ 230 - 2 . . .
, , .__ .
_ _, 3o :
- WO92/12010 ~ f~ PCr/US')2/0022l The heat seal performance o~ Resin B and Resin C prior to and after the radiation treatment was evaluated by the method described in Example 1. The results are illustrated in Figure~ 4 and 5 and the data are summarized in Table~ 5 and 6. The data lllu~trate that Resin C retained good heat seal performance (i.e., a broad temperature bonding window yielding good heat seal values) even after electron beam radiation treatment of up to about 4 Mrad of radiation do3age. As Figure 4 shows, after an irradiation exposure of about 4 Mrad, Resin C achieved a heat seal strength of about 10 Newtons/inch (4 Newtons/cm) or more at a heat seal temperature as low as about 195F (91C). Resin C is thus an example of this invention for use in the heat seal layer of a multilayer structure demonstrating good heat seal at low temperatures after irradiation.
However, for Resin B, heat seal performance deteriorated significantly. After about 4 Mrad irradiation dosage, Resin B achieved a heat seal strength of about 10 Newtons/inch (4 Newtons/cm) only after heat sealing at a temperature of about 215F
(102C) or more.
~ 30 WO 92~12010 2. .1 ~ ~ 3 ~ 3 P~/us92/no22l ~
~_ _ _ _ _ _-- _ _ C L ~ u E u ~ E u _ ._ L S X O ~9 Z ~ u rn Z J~ z ~ z o z rn z td r~ ~ _ _ _ _ ~_ H C C ___ _ _ _ _ _ _ _ _ 3 ~ E u IE E u E
l~n ~ L Z~~ O rJ Z ~ rZ rn Z ~ -2 ~ z rn rn Z
m ~ ~ . n r~ _ ~ rr) rn rr) ¢~ c~ _ _ _ . _ _ _ ~ ~ ~ ,~ _ _ ~r _ _ S S~L O U u ~r Z rn z rn Z u u ~o _ _ ~_ _, ~_ L D_ _ _ _ _ _ _ L :
~: ~ i 3 '~J ~
c ~ ~
a~ ~ . .= _ _ _ , .. _ _ c ~ ~ E ~ E E ~
L .C L O O O U~ U ~ ~ ~ ZID Z ~ Z
,' ~ ~ ~r ~ r~ ~ ~ ,u~
L ~: _ __ _ _ ~ O . _ ~ ~
' a) C 3 ~ O C~ O E E E ~tz ~ z ~ , ~ _ _ _ _ `,~ ¢,:: U~ ____ _ ___ __ :' G la ~ E ~ ~ ~ ~
~0 3~ L O O U) V o Z ~ Z ~ D ~ Z t~l Z
bO O ~ _ ~ ~J ~
,, ~.~--~ _ _ __ __ ___ _I L ~
,~ ' ~;,,.,~.. ".. , ", " ~,.. .... .. .
,~ ', WO92/12010 ~ PCr/US9~/00221l'-Example 3 This example demonstrates radiation crusslinking performance of LLDPE resins to be used for the core layer of the multi-layer film products with improved heat seal, orientation and shrink performance.
An octene-1 LLDPE copolymer, labeled a~ ~esin D, having I2 = 0.55 grams/10 minutes, I10/I2 : 12 and density of about o.g23 g/cm3 was fabricated into 1 mil (.025 mm) thick film using a blown fllm process. The blown film is made using a STERLING Blown Film Line.
This line has a 3.5 inch (8.9 cm) diameter extruder, L/D
of 30:1, an 8 inch (20 cm) diameter Western Polymer die with a 70 mil (l.8 mm) die gap, a STE~LEX barrier type screw and Internal Bubble Cooling (IBC).
Film samples made from this resin were treated by electron beam radiation using an Energy Science Incorpoated electron beam radiation machine (Model EC200; 200 KeV). The treated polymer film was evaluated for propensity for thermal shrinkag~! by measuring percent gel (i.e., crosslink density) by the ~olvent extraction technique as described in Example 2.
The results are illustrated in Figure 6 and the data are summarized and compared to Resin C in Table 7.
The data show that resin D had an enhanced radiation crosslinking efficiency as compared to Resin C (data shown earlier in Example 2, Table 4). Resin D, havlng I10/I2 of about 12, had a crossllnk density at a dosage of about 3 Mrad o~ approximately 18 to 20 percent gel, whereas reqln C does not attaln a cro~s-link density o~
about 20 percent gel until expo~ure to about 7 Mrad.
Resin D would, therefore, attain higher ~hrlnkage at ~ i, - - , 'WO92/12010 ~ 3 ~ 5 !) pcr/usl)2/oo22i lower levels of irradiation than resin C. and is thus an example of this invention for use as a core layer in a multilayer structure.
Crosslink Density of Resins C and D
af'ter Xrradiation :
Crosslink Density Radiation Dose ( Percent Gel) (Mrad) ~
Resin D Resin C
1 0 _ ~ __ O
__ . . . ... . ..... O
r 2 8 2 6 ~ 45 1 5 ___ 1 5 f~ 55 30 ~O 61 45 . 25 . .
.~ . .
, , .
. . .
WO9t/12010 PCr/U$~2/00221 Example 4 This example demonstrates a two layer co-extruded film product having lmproved heat ~eal, crosslinking and orientatlon/shrink performance after irradiation to 3 Mrad.
A 1 mil (0.025 mm) thick 2-layer (A/B) co-extruded film sample was fabricated by the following blown film process:
Two layer blown films were prepared in accordance with this invention by fabrication on an Egan coextrusion blown film line equipped with a 2.5 inch (6.4 cm) diameter main extruder, a 1.5 inch (3.8 cm) skin layer extruder and a 2 layer single air ring cooled annular die. The multilayer laminated film structure was an A/B configuration in which:
(I) The "A-layer" was the heat seal layer and was extruded through the outer annular die as the outer layer. The "A-layer" extruder was a 1.5 inch (3.8 cm) Egan having an L/D ratio of 20:1. The extruder had four zones temperature control. This extruder was used to ~eed ~esin C- to the outer layer. This layer wa~
maintained at a thickness of approximately 0.15 mils (0.0038 mm). The temperature profile of this extruder was as follows:
Zone 2 4 . --. .
Tem~erature 190 2Z0 230 220 .
. . .
;''' ' .
~, , -- W0~2/t2010 ~ 6 ~ PC~/U~92lO0221 The polymer melt temperature was about 225C. The throughput rate was about 18 pounds/hour (8.2 kg/hr).
(II) The "B-layer" ls the suppor~ or core layer and was extruded through the inner annular dle a3 the inside layer. The "B-layer" extruder was a 2.5 inch (6.35 cm) Egan having a screw L/D ratio of 24:1. This extruder is used to feed Resin D to the middle annular die to form the inner layer at a throughput of about 110 pounds/hour (50 kg/hr) and at a melt temperature of about 225C. The thiokness of this layer i~
approximately 0.85 mils ( 0.022 mm). The temperature profile of this extruder is as Pollows:
Temperature 205 220 230 220 -Resin C described in Example 2 was used to fabricate the heat seal layer and Resin D described in Example 3 was u~ed to fabricate the other layer. The thickness of the heat seal layer is approxlmately 15 ` percent of the total film thickness of about 1 mil (0.025 mm). The filrn sample is irradiated at 3 Mrad using an Energy Science Incorporated electron beam radiation machine (Model EC200). The total crosslink den~ity of the film, measured a~ percent gel, after the radiation treatment wa-~ approximately 16 percent. At cro~link den~itie~ of between about 10 percent and about 25 percent, the fLlm is known to have ldeal performance for orlentation and ~hrink applLcatlons.
." ,. . . .
~, ' '''i''"' , .
` wo 92,l20.2 ~ ~ ~ 3 ~ ~ Pcr/us92/oo221 ¢-, The heat seal performance (heat 3eal layer to heat seal layer) of this co-extruded cro~linked film was evaluated by the method described in xample 1.
Table 8 illustrates the heat seal strength of the heat seal layer of film before and after radiation treatment ,~ ~
; ' (3 Mrad). The film retained it~ excellent heat ~eal . performance after the electron beam radiation treatment.
Table 8 Heat Seal Strength of the Heat Seal Layer of Coextruded Film Samples before and after Irradiation 1 0 ~ . ~ , ....... ___ __ Heat Seal Strength Heat Seal (Newton~/lnch) ~:~Temperature (F) _ _ 0 Mrad 3 Mrad ~ , , . .
~; 15180 0 0 , . ... . . __ ~ _ .--:: 190 9 8 : (88C) (3.5 N/cm) (3.1 N/cm) , .,, , ., .. _ . _ :: (93C)(7.9 N/cm) (7.9 N/cm) : 20 . ~ _ _ (99C)(13.4 NJcm) (14.2 N/cm) . ,. __ ~
. 220 38 42 (104C3(15 N/cm) (16.5 N/cm) ,, __ _ _ . 25 :1 ` ' , ~ 30 , : .
, , 2 ~. t~
-~WO92/12010 PCI'/U~92/00221 This example illustrate.c~ that by uslng the resins di3closed in this inventlon for 3pe¢1flc layers.
a multi-layer film can be made, exposed to radiation and still maintain a combination of excellent core layer crosslinking performance and excellent heat seal layer performance.
The invention is particularly useful in heat-~, seal packaging of food. whereby the packaged food is ; ' 10 exposed to irradiation for sterilization purposes. The irradiation simultaneously strengthens the core layer and prepares it for shrinkage upon subsequent exposure ,.~r,:,~, to heat. By using the present invention in the heat seal outer layer, the heat seal initiation temperature of the heat seal layer is maintained after irradiation and the heat seal strength of the package is :
' i ,., ~
:: ~ 25 ,, ~ .
, :, 30 ~., i ,":, ', ' '''' ' "
,, . ,~, ~, . '- .
: ~ , ; ,, ,