CA1096127A

CA1096127A - Antistatic, low temperature heat sealable polypropylene composite film and process for production thereof

Info

Publication number: CA1096127A
Application number: CA277,277A
Authority: CA
Inventors: Mikio Tsuchiya; Takashi Kawamura; Kaoru Shibata
Original assignee: Honshu Seishi KK
Current assignee: Honshu Seishi KK
Priority date: 1976-07-22
Filing date: 1977-04-26
Publication date: 1981-02-24
Also published as: AU2430677A; JPS5312971A; JPS5524418B2; US4117193A; GB1571158A; AU505982B2

Abstract

Abstract of the Disclosure There is disclosed a novel composite film prepared by melt extrusion laminating a polymer blend composition com-prising a low-crystalline resin of ethylene-butene copolymer and a polyolefin resin having incorporated therein a lubricant and an anti-blocking agent onto surface or surfaces of a uni-axially stretched polypropylene film followed by stretching the laminate film in the direction perpendicular to the direction in which said polypropylene has been stretched and optionally subjecting the resulting film to corona discharge treatment.
The composite film thus made provides an excellent packaging material because of its improved antistatic, lubricating and anti-blocking properties in addition to its heat sealability feasible at lower temperatures than prior art materials.

Description

10~6~27 ~ his invention relates to an anti-static~ self-lubricating and low-temperature heat sealable polypropylene composite film and a process of preparing the sameO
It is common knowledge that crystalline polypropylene biaxially oriented film has numerous uses as packaging mate-rial, particularly as food packaging material because of its various exceptior~al properties including excellen-t optical properties such as transparency, brilliance, etc O; excellent mechanical properties such as tensile strength, Young's modulus, etc~; very small. vapor-permeability; non-toxicity and non-odorous properties; and the likeO However~ it has a defect in that the single substance film thereof has rather poor heat sealability, and even if it is heated -to a tempera-ture at which heat sealing is feasible, satisfactory sealing strength cannot be obtained, and in addition, it is easy to produce buckling due to heat shrinkage which spoils the beauty of the sealed surface and reduce the value of film as a com-mercial article~
To overcome such defects~ a number of attempts for improvement have been tried hitherto~ Represen-tati~e examples of such attempts include (~) a process which comprises pre-paring a solution or an emulsion from easy heat sealable resin and then applying the resulting solution or emulsion to one or both surfaces of -the biaxially oriented polypropylene film;

(2) dry lamination which comprises coa-ting an adhesive on one or both surfaces of the biaxially oriented polypropylene film and then laminating thereon easily heat sealed resin film or films; (3) a process using no adhesive which comprises melt extrusion laminating at a high temperature easily heat sealed resin onto one or both surfaces of the biaxially oriented film;

~k ~96~Z7 and -the like~
~ he process (1) mentioned above has defects in that a number of processing steps such as a coating step, a drying step and a solvent-recovering step are required, and in addi-tion, since these steps cannot be carried out in a throughprocess, cost of the product increasesO Also, sealing strength is at most in the region of 200 - 300 g/25 mm, which is satis-factory only for simple packagingO
~he process ~2) mentioned above also requires a separate step for applying adhesives to the polymer film, and therefore is not free from the defect that cost of the product increases.
~ he process (~) mentioned above does not require the steps of applying solvent or adhesives, but this process must also be carried out separately from the process of making the polymer filmO Also, to provide a very thin laminate coating such as in the region of 10 ~ or so by this high temperature melt extrusion coating process is extremely difficult from a technical viewpointO
As a result of many studies which have been made to elimina-te these defects of the prior art processes, -the pre-sent inventors have accomplished the invention of a composite film which has e~cellent anti-sta-tic proper-ties~ excellent lubrica-ting properties and low-temperature heat sealabilityO
This composite film of the present invention has additional advantages in that cost of the product i3 decreased and the film can be used in such processes as high-speed, automatic bag making or high-speed, automatic packaging processO
The composite film of the present invention is prepared by melt extrusion laminating a polymer blend composition com-prising (a) a polymer blend of a low-crystalline resin of il~;J)96127 ethy:Lene-butene copol-~mer arld a polyolefin resin; and (b) addi-tives; a lubricant and an anti-blocking agent; onto at leas-t one surface of a uniaxially stretched pol~rpropylene film prepared by uniaxially stretching a polymer composition comprising (a) a propylene homopolymer or a propylene copolymer containing a small amount o~ ~-olefin and (b) additives 7 a lubricant, an anti-blocking agent and an antis-ta.-tic agent and then stretching the resulting laminate film in the direction perpendicular to the di~ection in which said polypropylene film has been stretchedO ~he biaxially oriented film thus prepared is preferably subjected to corona discharge treat-ment to further improve the properties of the composite filmO
The polypropylene from which said uniaxially stretched film substrate layer is prepared may include a propylene homopolymer and a propylene copolymer containing upjto 5% by weight of ethyleneO The polyolefin resin as one component of the polymer ~end mentioned above is preferably a high density polyethylene (namely, low~pressure-processed or moderate-pressure-processed polyethylene)D The ethylene butene copolymer as ano-ther com-ponent of the polymer blend mentioned above can be a copolymer compri.sing 50 - 99% by weigh-t oY ethylene and, correspondingly, 50 ~ 1% by weight of butene. More preferably, this e-thylene~
butene copolymer comprises 90 - 99% by weight of e-tkylene and, correspondingly, 10 - 1% by weight of buteneO The polymer blend which can be used as a laminating resin in the practice of the present invention can cornprise 20 - 60~ by weight of said ethylene-bu-tene copolymer and, correspondingly, 80 -L~0% by weight oY said polyethyleneD More preferably, the polymer blend comprises 25 45% by weight of -the same ethylene-butene copolymer and~ correspondingly, 75 - 55% by weight of ~D9~i~27 the same polyethyleneO The composite film of the present invention generc~lly has a -thickne.ss in the region of 10 - 60 microns, more preferably, in the region of 15 - 50 microns, and most preferably, in the re~ion of 20 - 30 micronsO
In the following description, the processes through ~hich the present invention has been accomplished will be explainedO
~he present inventors have found -that biaxiall~
stretched polypropylene GompOsite film which is prepared by applying a low-crystalline ethylene-butene copolymer ~ one or both surfaces of a crystalline polypropylene uniaxiall~7 stretched film containing an anti-static agent 7 a lubricant and an anti-blocking agent by melt extrusion laminating method, namely without any pre-treatment such as the application of adhesives, and then stretching the resulting laminate film in the direction perpendicular to that in which said substrate film has been stretched and finally subjecting at least one surface of sai~ laminate film to corona discharge treatmen~
is one having lamination layer 3 ,u thick and cxhibits heat seal stre~gth at least in the region of 600 - 800 g/25 mm (a-t 110C), anti-static properties, in -t~rms of "ratio of electrostatic decay", in the region of 80 - 100~o and surface resistivity in the region of 101 - 1011 cmO
~owever, though this ethylene-butene copolymer resin composite film is excellent in its transparency and low-temperature heat sealability~ it has a defect in that since ethylene-bu-tene copolymer resin has properties intermediate between rubber and plastic, the film made of it is poor in its lubricating properties and anti-blocking properties.
~he present inventors have also found -tha-t by blending ~961Z7 in said. ethylene-bu-tene copol~ner resin a polyolcfin-t~-pe thermoplastic resin, particularly a high density polyethylene (or a moderate- or low-pressure-processed polyethylene resin) having compatibility therewith~ the above-men-tioned defects of ethylene-butene copolymer resin can be elimi.nated, and in addition, the essential properties of said resin cc~ be pro-motedO
~he attempt to irrlprove lubriccating and anti-blocking properties of ethylene-bu-tene copolymer resin by simply in-corporating therein, as a lubrica-ting agent, 0 2 - 10~/o by weight of a conventio~al higher fatty acic. arnide (such as, for example, oleic amide, stearic amide, erucic amide and the like) and, as an anti-blocking agent, 001 - 0O5% by weight of silica will be successful to a certain level, because the added amide bleeds to the surface of the low-crystallinity ethylene-butene copolymer resin layer and there-by the lubricating properties of said resin are improved an~' at the sarne time the anti-blocking properties, at least, of the same resin are substantially ir~provedO However, said attempt will not completely avoid the blocking of a rolled film irnmediately after the formation -thereofO ~o increase the amounts of these lubricating and anti-bloc~ing agents generally leads to the spoiling of the appearance, which de-creases the value of the~y~ s an articleO
Hereupon, the present inventors have tried to blend with said ethylene-butene copolymer resin an olefin type resin, particularly a high density polyethylene (namely, a moderate- or low-pressure polyethylene) which originally has good lubricating and anti-blocking properties and has good compatibility with ethylene-butene copolymer resinO lt has been found that when 40 - 80,h by weigh-t, preferably 55 - 75%
by weight based on the total amount of blending components, of said olefin t~7pe resin i5 blended, -the lubricating and anti-blocking propcrties of the e-thylene-butene copolymer resin are substantially improved and the blocking in the roll film can also be avoidedO It has also been found that if ordinary lubricating agent (such as amides of highe~ ~atty acids) in an amo~lt of 0~1 - 1O 0% by weight, and an anti-blocking agent (such as silica) in an amount of 0005 - 002,~
by weight are additionally incorporated, extremely good lubricating and anti-blocking properties can be obtainedO
~urther, it has also been found that blending a polyolefin-type resin, preferably high density (or moderate- or low-pressure) polyethylene into ethylene-butene copolymer resin not only eliminates the above-mentioned defects of the latter resin but also positively improves the desirable properties which the same resin originally possessesO ~ha-t is, the fol-lowing additional advantages are observedn Ethylene-butene copolymer resin is generally heat sealable at a low temperature of 110C and heat seal strength at tha-t temperature is in the region of 500 - 600 g/25 mm.
In contrast, if polyethylene, preferably high density (or moderate- or low-temperature) polyethylene(for example, of 0O94 g/cm3 density) is blended therein, the modified resin shows a heat seal strength of 800 g/25 mm or higher and in addi-tion, as a result of the hardness of resin being improved both scratch resistance and stiffness as a co~.posite film are improvedO
~he improved anti-static properties C&~ be obtained in accordance with the present invcntion by incorporating l~g~2~7 002 - 100% by weight o~ an ordinary anti-sta-tic agent only in substrate polypropylene film layer and then subjecting at least one surfnce of the stretched composite film to corona discharge treatment to obtain the surface tension of 35 dyns/cm or g~eater, preferably in the region of 35 - 45 dyns/cm, most preferably in the region of ~7 - 40 dyns/cm~
In this type of composite film, an anti-static agent is generally incorpora-ted in a lamination layer and no anti-sta-tic agent is incorporated in a substrate layerO Accord-ingly, it is surprising to find that both antistatic and anti-blocking properties of such composite can substantia~Lly be improved by incorporating an antistatic agent in a substrate layer, omitting to incorporate it in a lamination layer, in accordance with -the present invention~
In the following lines, the results of investigations with respect to the technical requirements in the practice of the presen-t invention will. be explainedO
~ffect of ~he Amount of a ~ubricating A~ent to be In-corporated in a La inating Resi.n An isotactic polypropylene resin ~MFI-2.0) having incorporated therein 0~1% by weight of a lubricati.ng a~en-t (stearic amide),005~0 by weight of an anti-b:Locking agent (silica) and 0.~% by weight of an an-tistatic agent (N,~-bis(2-hydroxyethyl) alkoxypropylbetaine) was prepared to be used as a substrate layerg and it was uniaxially stretched by five times to obtain a uniaxially stretched film 235 u thick by using the sequential biaxially stretching apparatus~
To a blend comprising 30 parts by weight of ethylene-butene copolymer resin and 70 parts by weight of hi.gh-pressure (namely, low density) polyethylene were added 0~15/ by weight ~9~i127 of an anti-blocking agent (silica) and sig di~ferent levels, vQrying from 002 to 0065% by weight~ of a lubricating agent (oleic amide) followed by extrusion melt blending these com-ponents to obtain the pellets of laminating resinO ~heng the laminating resin was applied onto one surface of said uniaxially stretched film (substrate layer) to provide thereon a lamination layer 35 ~u thicko Then, the resulting composite film was stretched in the direction perpendicular to that in which said uniaxially-stretched film had been stretched by nine times to finally obtain the composite film ~0 ,u thicko ~he non-laminated side surface of the composite film was subjected to corona discharge treatment to the degree of 40 dyns/cmO
The properties of the resulting composite film are as shown in Table 1 belowO The table shows that as the per-cent addition of lubricant increases 7 the haze increases, while the coefficient of dynamic friction decreasesO ~hese results can be illustrated by a diagram as shown in Figo 1.
The results of Table 1 show -that any amount in the region of 002 - 0~65% by weight is effective to the advantage of the present inven-tion, so long as it concerns the addition of a lubricantO In the practice of the presen-t invention, the amount of addition can be in the region of 0O1 - ~
From the viewpoint of both lubricating proper-ties and trans-parencyg the preferred amount of addition is in the region of 0.2 - 0O~% by weight~

~g612~

Table Lubricat- I , (1) Coef- 1l (2) Ratio ¦ (~) Anti- I l1eat ing Agent I ~-T z (~,)j ficient of I of Electro-,,blocking j Seal add~d (~/ I a e ~ I dynamic I static jPropertiesl Strength by weight) , friction I decay (%) I (Class) (g/25mm) 002 1 108 0O45 1l100 ! 1-2 1 700 0~3 1l 2~3 1 0O30 1100 ¦ 1-2 ¦ 700 0~4 ll 207 j 0O23 ~90 ~ 700 0O45 ~ 0O20 ~85 ~ 1 1 700 0O5 ¦ 3O3 0019 ¦80 ~ 1 1 700 0O65 ~ 4O0 ~ 0~17 ~80 1 ~ 1 700 I ~
Notes. (1) Laminated surface x non-lc~linated surface (2) ~on-laminated surface

(3) I,aminated surface x non-] aminated surface Effect of Chanp~e in Ratio of_the Thickn ss of the Uni-axiall~ Stretched ~ilm (Substrate Film) to the ~h_cness of the I,aminatio.n Icqyer .
The ratio of the thickness of a substrate layer to -that of a lamination layer was varied -to determine how the properties of the composite film change with a change in their re] ative thlckness~
Using a method similar to tha~t described in the pre-vious item in w:hich -the effect of the amount of a lubricating agent was examined, 0O5% by weight of a l~.bricating agent (stearic amide) and 0~15% by7 weight of an ani,i-blocking agent (silica) were inco.rporated in a combination of 40 parts by weight of ethylene-butene copolymer resin and 60 parts by weight of high-pressure polyethylene and they were blended by a melt extrusion process into pelle-t.s which would be used as a laminating resi.nO The same additives as mentioned cabove 10~612~7 were incorporated in the isotac-tic polypropylene resin men-tioned above and the resulting resin was stretched to five times its, original length to a thickness within the range of 200 - 260 ,u to provid.e uniaxially stretched films ~of 4 different thicknesses) which would be used as a substrate layerO ~hen9 the laminating resin was applied onto one surface each of said substrate layers -to provide there a lamination layer in -the region of 72 - 18 ,u (4 levels) thick~
~hen, each of -the composite films was stretched to nine times its original length in the direction perpendicular to that in which the substrate layer had been stretched in order to form the final composite film 30 ,u thicko ~hen, corona discharge treatment was applied to the non-laminated side surface of each of said composite films -to eventually pro~-ide the surface tension of 40 dyns/cmO The properties of the resulting composite film are as shown in ~able 20 When the thickness of the four laminated resin layers is within the range of 72 - 18 ~ (correspondingly within the range of 8 - 2 ,u after stretching~, it was observed that as the -thickness of the laminated layer increases, -the ratio of electrostatic decay tends -to decrease~ and the heat seal strength increases, while the other factors show almost no substantial changesO Based on these results, one can determine -the most desirable thic~ness of lamination f`or each purpose, taking the haze, the ra-tio of electrostatic decay, the heat seal strength, etcO into considerationO

lOg6~7 Table 2 Thicknes ~ Coeffici- Ra-tio of An-ti-substra-te ¦ ~I ent of electro- blocklng Heat seal layer/lami-l (aO~Z)e dynamic static proper- strength nated layer friction decay ~) (Class) (110C) . _ 28/2 200 002-Oo3 100 1-2 600 26/4 2~3 0~2-Oo3 95 1-2 7oo 24/6 205 002-Oo3 80 ~-2 800 22/~ 205 0~2-003 80 1-2 (5) i _ _ _ _ i _ 800 Notes (1) Laminated surface x non-laminated surface (2) Non-laminated surface (3) Laminated surface x non-laminated surface

(4) Laminated surface x non-laminated suxface

(5) Peeling could not be finished bef~re the rup-ture of one of the test pieces of film Brief ~xplanation of the Drawlngs:
Figo 1 shows the effect of the addition of a lubricat-ing agent to a laminating resin;
~ ig~ 2 shows the effect of the incroporation of poly-ethylene in accordance with the present invention on theheat sealing suitability of the filmO
Aooo laminated composite film comprising ethylene-butene copolymer (40 parts) and modexate- or low-pressure polyethylene (60 parts) Booo composite film laminated wi-th ethylene-butene copolymer resinO
~ he following comparative and working examples will explain the present invention furtherO In these examples, 10~ 7 the measurements were obtained by the following methodsO
(1) Haze The haze was determined by the methods defined in JIS K-671~ and K-6718 using a haze meter available from ~IHO~ DENSHOKU KOGYO COO9 L~Do (2) Lubricating properties (Coefficients of static-and d~namic frictions) It was determined by the method defined in AS~M D-1~94 (B-method) using ~ENSILONavailable from ~OYO SEIKI COn ~ I~TDo (3) An-ti-static properties Ratio of electrostatic decay (%): '~he film was left for 24 hours in an atmosphere having a tempera-ture of 20C and 65% RHo After that, it was charged at 6 KV for 1 minute using "Static Honestmeter" made by SHISHIDO SHOKAI COO, ~q'D., and then discharged for 30 seconds before said ratio was determinedO
Surface resistivit (Q cm) ~he film was left for 24 hours in an atmosphere having 20C and 65/~ RHo ~hen, the surface resistivity thereof was measured b~ the "Super Insula-tion ~ester "SM-10" of ~OA DEMPA COO~ L~Do makeO
(4) Eeat seal strength Onto -the contacting surfaces to be heat sealed with each other~ each 25 mm wide was applied pressure of 2 Kg/cm2 for 1 second b~ "Multi-temperature Level Heat Sealer" available from ~OYO SE`IKI ~0~, L~D~ and the resulting heat-scaled surfaces were left for 24 hours in an a-tmosphere having 20C and 65% RHo After that, the strength of force required for peeling was ~6~ 27 measured at a rate of 300 mm/min with "TE~SILO~" of TOYO S~IKI COO 9 I~TDo makeO
(5) ~lti-blocking proper-ties Test pieces of film each 50 mm x 50 mm in size were prepared by random sampling and two of them were placed in contact with each other in such a way that the surfaces to be examined were face to face. The two strips of film were left in an atmosphere of 40C
and 90~,/o RH for 24 hours under applied pressure of 2 Kgso After that~ the two strips of film were examined and evaluated in terms of the degree of blocking observed, the ranking ranging from Class 1 (no blocking) to Class 5 (complete blocking)O
Comparative ~xample An isotactic polypropylene resin (M~ I = 200) having incorporated therein 001yo by weight of a lubricating agen-t (stearic amide), 005% by weig~ht of ar arti-blocking agent (silica) and 005% by weight of an anti-static agent was stretched to five times i-ts original length by using the sequential biaxially stretching ap~aratus to obtain uniaxiall~ s-tretched film 235 ,u thicko To 100 parts of e-thylene-butene copolymer resin were added 004/O by weight of luhricating agent (oleic amide) and 0015% by weigh-t of an anti-blocking agent (silica) to obtain a laminating resinO Then, the laminating resin was applied onto one surface of said uniaxially stretched film to provide thereon a lamination layer 35 ~ -thicko Then, the resulting composite film was stretched to.nine tilles its original length in the direction perpendicular to that in - which said uniaxially stretched film had been s-tretched to finally obtain a composite film 30 ~ -thicko The non-lamina-ted l~g~;12~7 surface of said composite film was subjected -to corona dis-charge treatment to the de~ree of 40 dyns/cmO
The properties of the composite film thus obtainecl are as shown in Table 3 and Table 4 belowO

~9G12~

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___ i~g~lZ~7 Example 1 A composite film 30 u thick was prepared in ~he same manner as in the Comparative Example by applying the same laminating resin as used in said Comparative Example except that instead of 100 parts of the ethylene-butene copolymer resin a blend consisting o~ 40 parts by weight of the same copolymer resin and 60 parts by weight of high-pressure polyethylene was applied to one surface of the uniaxially stretched film prepared in the same manner as in the same Comparative ExampleO
: Properties of the composi-te film thus obtained are given in Table 3 and ~able 4 together with the conditions under which the composite film was prep.aredO
_xample 2 Onto one surface of the uniaxially stretched film prepared entirely in the same manner as in the Comparative Example was applied the same laminating resin as used in the Comparative Example except that instead of 100 parts by weight of the ethylene-butene copolymer a blend consisting ~: 20 of 30 parts by weight of the same copolymer resin and 70 parts by weight of high-pressure polyethylene were used, said laminating resin having incorporated therein the same addi-tives as in the above-mentioned Comparative Example~ Thus, the composite film 30 ,u thick was prepared in the same mannerO
Properties of the composite film thus obtained are given in Table 3 and Table 4 together with the conditions : under which it was preparedO
:~ Example ~
Onto both surfaces of a uniaxially stretched film : 30 150 ,u thick prepared in the same manner as in the Comparative - 1~ -Example was applied a laminating resin comprising 40 parts of e-thylene-butene copolymer and 60 parts of high-pressure (low density) polyethylene and having incorporated thercin as additives 005% by weight of a lubricating agent (stearic amide) and 0015% by weight of an anti-blocking agent to the thickness of 15 ,u, respectively, for both sides and.further according to the same manner as in the Comparative Example the composite film finally 20 ,u thick was pr.epared. One of said surfaces was subjected to corona discharge treatment to the degree of 37 dyns/cm.
~he results are as shown in ~able 3 and Table 40 Example 4 ; Onto a uniaxially stretched film 150 ,u thick prepared in the same manner as in the Comparative Example was applied a laminating resin comprising a blend of 40 parts of ethylene-butene copolymer resin, and 60 parts of high density (moderate-or low-pressure) polyethylene having incorporated therein 005%
by weight of a lubricating agent (oleic amide) and 0015% by weight of an anti-blocking agent (silica) in -the same manner as in ~xample 3 to finally obtain a composi-te film 20 ~ thic~o Corona discharge treatment was applied therc-to in the same : mamler as in Example 3.
Preparing conditions and properties of the resulting cpmposite film are as given in ~able 3 and ~able 4.
As is obvious from ~able 3 and ~able 4, improvement in "coefficient of dynamic friction" (lubricating properties) and anti-blocking properties is observed with respect to Examples 1 and 2 as compared with the ~omparative Example (one-surface lamination)O Results of two-surface lamination of the Comparative Example are not shown in said tablesO

_ ~9 _ l~g6lzt7 No substantial anti-blocking properties were observedO It is difficult to make articles therefromO If polyethylene is blended in the ethylene-butene copolymer resin as in ~xamples 3 and 4, the production of improved composite film having substantial anti-blocking properties and satisfactory low-temperature heat seal strength is ensuredO
Comparison of heat seal suitability of the composite ~ film of the present invention obtained in ~xample ~ and the composite film obtained in the Comparative ~xample is shown in Figo 20 What is evident on comparing the two curves is that one which represents the heat seal strength of the com-posite film of ~xample 4 has a steeper risingO
By using the film prepared in accordance with the present invention and the automatic packaging machine (~ype ~ 15 W-~7 available from TOKYO JIDO KIK~I COO, ~Do ~ packaging ; test for applying said film to marketing packs of cigarettes was carried out at a packaging rate of 150 packs/minuteO
Satisfactory packaging suitability of the film was proved.
`: Heat sealing was feasible at a temperature 10 - 15C lower , as compared with the case in which ordinary moisture-proof cellophane was used, with the heat seal strength bcing excel-lentO

~, , '`

Claims

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

1. An antistatic, low-temperature heat sealable poly-propylene composite film which comprises:
(a) a substrate layer of a uniaxially stretched film of a polymer composition comprising a propylene homopolymer or a propylene copolymer containing up to 5% by weight of alpha-olefin, said polymer composition having incorporated therein (i) a lubricant, (ii) an anti-blocking agent, and (iii) an antistatic agent; and (b) a lamination layer or layers of a polymer blend composition comprising a low-crystalline resin of ethylene-butene copolymer and a polyolefin resin laminated on at least one sur-face of said substrate layer, said polymer blend composition having incorporated therein (i) a lubricant and (ii) an anti-blocking agent, said composite film having been stretched, after lamination, in the direction perpendicular to the direction in which said substrate layer has been stretched.

2. The composite film of Claim 1 which has been subjec-ted to corona discharge treatment after the final stretching.

3. The composite film of Claim 2 in which the corona discharge treatment has been carried out until the surface tension of the treated surface reaches 35 - 45 dyns/cm.

4. The composite film of Claim 2 in which the corona discharge treatment has been carried out until the surface tension of the treated surface reaches 37 - 40 dyns/cm.

5. The composite film of Claim 1 in which said poly-propylene in the uniaxially stretched film substrate layer is selected from the group consisting of a propylene homo-polymer and a propylene copolymer containing up to 5% by weight of ethylene.

6. The composite film of Claim 1 in which said polyolefin resin in the polymer blend in the lamination layer is a high density polyethylene.

7. The composite film of Claim 1 in which said ethylene-butene copolymer in the polymer blend in the lamination layer is a copolymer comprising 50 - 99% by weight of ethylene and 50 - 1% by weight of butene.

8. The composite film of Claim 1 in which said ethylene-butene copolymer in the polymer blend in the lamination layer is a copolymer comprising 90 - 99% by weight of ethylene and 10 - 1% by weight of butene.

9. The composite film of Claim 1 in which said polymer blend in the lamination layer comprises 20 - 60% by weight of the ethylene-butene copolymer and 80 - 40% by weight of the polyethylene.

10. The composite film of Claim 1 in which said polymer blend in the lamination layer comprises 25 - 45% by weight of the ethylene-butene copolymer and 75 - 55% by weight of the polyethylene.

11. The composite film of Claim 1 having a thickness in the region of 10 - 60 microns.

12. The composite film of Claim 1 having a thickness in the region of 15 - 50 microns.

13. The composite film of Claim 1 having a thickness in the region of 20 - 30 microns.

14. A process for the production of an antistatic, low-temperature heat sealable polypropylene composite film which comprises the steps of:
melt extrusion laminating a polymer blend composition comprising (a) a polymer blend of (i) a low-crystalline resin of ethylene-butene copolymer and (ii) a polyolefin resin; and (b) additives: (i) a lubricant and (ii) an anti-blocking agent; onto at least one surface of a uniaxially stretched poly-propylene film prepared by uniaxially stretching a polymer com-position comprising (a) a propylene homopolymer or a propylene copolymer containing up to 5% by weight of alpha-olefin and (b) additives: (i) a lubricant, (ii) an anti-blocking agent and (iii) an antistatic agent; and then stretching the resulting laminate film in the direc-tion perpendicular to the direction in which said polypropylene film has been stretched.

15. The process of Claim 14 which includes an additional step of subjecting the biaxially stretched film to corona discharge treatment.

16. The process of Claim 15 in which the corona discharge treatment is carried out until the surface tension of the treated surface reaches 35 - 45 dyns/cm.

17. The process of Claim 15 in which the corona discharge treatment is carried out until the surface tension of the treated surface reaches 37 - 40 dyns/cm.

18. The process of Claim 14 in which said polypropylene in the uniaxially stretched film substrate layer is selected from the group consisting of a propylene homopolymer and a propylene copolymer containing up to 5% by weight of ethylene.

19. The process of Claim 14 in which said polyolefin resin in the polymer blend in the lamination layer is a high density polyethylene.

20. The process of Claim 14 in which said ethylene-butene copolymer in the polymer blend in the lamination layer is a copolymer comprising 50 - 99% by weight of ethylene and 50 -1% by weight of butene.

21. The process of Claim 14 in which said ethylene-butene copolymer in the polymer blend in the lamination layer is a copolymer comprising 90 - 99% by weight of ethylene and 10 -1% by weight of butene.

22. The process of Claim 14 in which said polymer blend in the lamination layer comprises 20 - 60% by weight of the ethylene-butene copolymer and 80 - 40% by weight of the poly-ethylene.

23. The process of Claim 14 in which said polymer blend in the lamination layer comprises 25 - 45% by weight of the polyethylene.