CA2104864C

CA2104864C - Thermoplastic film and method of welding same

Info

Publication number: CA2104864C
Application number: CA002104864A
Authority: CA
Inventors: Claude Desmarais; Richard Desmarais
Original assignee: Desmarais and Frere Ltd
Current assignee: Albums DF Ltd
Priority date: 1991-03-22
Filing date: 1992-03-20
Publication date: 1997-03-18
Anticipated expiration: 2012-03-20
Also published as: WO1992016358A1; CA2104864A1; USRE35991E; AU1565792A; US5449428A

Abstract

A method of welding at least two layers of a thermoplastic film which comprises a copolymer or copolymer-polymer mix-ture selected from the group consisting of ethylenevinylacetate (EVA) copolymer, an EVA copolymer and polyethylene, and EVA
copolymer and polypropylene, and an EVA copolymer and polyethylene and polypropylene, with the proviso that the concentra-tion of vinylacetate in said film is from about 12 to about 28 % by weight, said method comprising the steps of: pressing said at least two layers of said thermoplastic film; applying a high frequency current to said pressed layers in order to weld said layers together; and recovering said welded layers.

Description

WO 92116358 2 1 ~ 4 8 6 ~ PCT/CA92/00125 I
J~hAsTIc FILM AND METHOD OF WELDING SA~IE
FIE;LD OF SHE INVENTION
The present invention is directed to a method of welding, by high frequency or ultrasonic vibrations, an ~,..~;,, "~ friendly ~h . "",~
S material, said material being ~d~ lJhllly suitable as a 1~,6~~ lt for high frequency welding of polyvinyl chloride containing materials.
BACKGROUND OF THI~ INVENTION
In the past decade, the concern of ~U._.IIIII~,Ilt:~ for ecology and Cll~.. J.. _Ill has become one of Ihe top, if not the top priority. In the United Stat, 80% of aU trash is being landfilled, the remaining 20% being split between i~l.;...~ldtiOII and recycling. However, landfilling has its limits, and soon, incineration and recycling will become the best, if not the only way of disposing of trash.
Go._,l,.ll~,llts throughout the world have enacted legislation concerning the use and disposal of wastes, and it is likely that upcoming i., ' laws will be more strict and rigid against any kind of pollution.
Accordingly, the great challenge of industries in almost every field is to keep on producing goods at reasonable costs, while meeting criteria more strict and the plastic industry is no exception.
At present, many items ' ~.1 from plastic materials are generally made of polyvinyl chloride (PVC). PVC is a versatile material, but hasserious .,..~ilulllll.,.lldl drawbacks. Il~ .tion of PVC produc chlorinated ....,.,r."~.,c~ chlorinated rlih~on7n~l ' i, and ll,~ acid gas. It is now of common knowledge that dioxins and furans have been linked to diseases such as immuno-system failure, cancer, and birth defects. Also, llydl~lllolic acid is a 30 powerful corrosive substance, and when combined with dL--l~ moisture, it S;~li~ Lly contributes to acid rain.

WO 92/16358 PCl'/CA92/00125 2i0486~ 2 Since PVC materials also contain toxic additives such as heavy metals used as stabilizers and plasticizers, disposal of these coverings by landfilling is highly undesirable because these additives are slowly leached by water, and thus dispersed in the soil. Most of these stabilizers are considered ~~
s The use of these stabilizers has also led to bans on PVC ' .~liull, and has raised questions concerning the acceptance of products containing PVC that could eventually end up in waste.
Many ISU~ have already banned either production and/or , IdliOI~ of PVC materials in their countries and this has lead to a race between Gl ~ to find a non-toxic substitute for PVC which can be welded at high frequency. This represents a tremendous task. Plastic manufacturers are investing important amounts of money at every level to develop a new th.. I.nJ~ lic material 15 suitable for high frequency welding which would overcome all the serious drawbacks of PVC. In this research, many ~cu,uilclllC~ must be met.
The new ~ , ' ~ material must be produced LUIYI~ .,IJ ~hl~
meeting the desirable u- . ~ .. e ,cu,uiu~...~..l~. The process leading to the final0 product should be susceptible to efficient continuous production operations. The material should also be strong enough to withstand stretching without the creation of holes, tears, or non-uniform zones of stretching.
The processing of PVC or similar materials into sheets involves 25 extremely expensive machinery and the challenge is to develop a ~
material which can, on one hand, be processed into sheets with existing machinery, and on the other hand, be converted into consumer goods by using the currently available machinery, especially such machines which use high frequency welding methods. Generally, the PVC materials are welded by high frequency, and the 30 replacing lll~lllupl~alic material must also be capable of being welded by high frequency in any subsequent operation, and that represents the key step leading to the final~consumer good.
.-..*~

WO 92/16358 21~ PCT~CA92/00125 Fiaally, the t~ luula~, material should be easily recyclable, or in the alternative, should cause little damage to the CIIV;II when incinerated.
The present invention fulfils all the above ICUUh~ -In accordance with the present invention, there is provided a methodof welding at least two layers of a lh.,~ vluL~ . f Im which comprises a copolymer or copolymer-polymer mixture selected from the group consisting of 10 ethyl.,l.~,.;."' ~ (EVA) copolymer, an EVA copolymer and polyethylene, an EVA copolymer and p~ uujl~llc, and an EVA copolymer and PUI~ llG and pul/,u~u,u~L,ac, with the proviso that the of v;~ a~elalG in said film is from about 12 to about 28% by weight, said method comprising the steps of:
- pressing said at least two layers of said Ill.,llllO~lla"i filrn;
- applying a high frequency current to said pressed layers in order to weld said layers together; and - recovering said welded layers.
For UUli.~ purposes, the above described film will be referred to as EVA film throughout the In an aspect of the invention, the EVA film comprises an cL~l~h.l.L. ~' ~ copolymer (EVA) and a polymer selected from the group consisting of pul~ ~lh~!~,l,c (PE), po~.. u~l~,..c (PP) and mixtures thereof, with the proviso that the ...ll~ tldtiOl~ of ~ a~etdtG in the film is from about 12 to about 25 28% by weight.
The EVA film of the present invention can be used as a suitable IG~Ia.Glll~,lll for PVC in the manufacture of goods using high frequency weldjngequipment in the llall~rullllatiOll process.
Preferably, the amount of ~ .LGldlC in the EVA film is from about 14% by weight to about 25% by weight and more preferably, about 19% by weight.

WO 92/16358 ~ ~ ~ PCr/CA92/00125 In ano~her aspect of the present invention, the layers of the i. film described above can be welded by ultrasûnic vibrations.
The EVA film used in the methûd of the present invention may alsû
5 contain one or more additives such as a colour concentrate, a slip, a filler, an anti-blûck agent and the like.
~ET~II Fn ~ESCRIPTION OF THE INVENTION
In accordance with the present inventiûn~ there is provided a method of welding at least two layers of a th... ~l~u~ ie film which comprises a copolymer or copûlymer-polymer mixture selected from the group consisting of eLl~ dcel~te ~EVA) copolymer, an EVA copolymer and pUl~ l. llC, an EVA copolymer and polypropylene, and an EVA copolymer and pol~.,ll.~l...c and poly,~,lu~ , with the proviso that the CUll~.,llllC~iUII of ~ ^ in said film is from about 12 to about 28% by weight, said method comprising the steps of:
- pressing said at least two layers of said l;~.,llllupl~lalic filrn;
- applying a high frequency current to said pressed layers in order to weld said layers together; and - recovering said welded layers.
The term ~/ol~ "c! is meant to indude high density ,uol~.,ll,~.~"lc (HDPE), low density pu~ .lc (LDPE) and linear low density pûlyethylene (LLDPE) and the like, with the LLDPE being preferred.
The term ~,uul,r~,u,uyl~ is meant to include ,uul,y,ulu,uyl~,llc l~OIIIul~u~ ,uu~ u~u~ copolymer~pc~ )lu~u~ lcterpolymerandthelike~with the pûl,y,ulu~u~ lc terpolymer being preferred.
.
The expression "high frequency welding~ is ayll~llJ~lluua of radio frequency welding, and the meaning of these expressions is well known in this art.
I~e wavelength used in the high frequency welding, or . ~diuL eu,u.,~l~ welding is of about 27 MHz.

WO92/16358 21~ ~ 8 ~ ~ pcr/cAs2~ool2s All the polymers or copolymers used in the method of the invention are commercially available on the market. As an example of such polymers that may be mentioned: linear low density polyethylene polymer #IIDI, ~ uL~lulc;l and sold by Dupollt canada; c~ J~ eL~le copolymer #28o3M~ ;l and S sold by At Plastics; pvlJ~Jlu~uJl~-lle hu~l~ul~GI~.~,~ JY 6100, r cd and sold by Shell Canada; pul~JluyyL~l.c copolyrner XSV 622, r ' cJ and sold by Himont; I~v~ v,uyl~ e terpolymer RAû61, l.,~ L.,Lu~cv and sold by Himont; low density pc,l~ hJ!u..c, r ' cd and sold by Nova Corporation under the trade mark NOVAPOL 2~0219-A; and high density p~ hJlc..e HDPE 36156, u~ ,uL.,iul~d and sold by Dow Chemical.
As an example of suitable EVA f Ims that can be used in the method of the present invention, there may be mentioned those comprising from about 42 to 100% by weight of EVA copolymer having a ~ ' content of 28% by weight, about 0 to 58% by weight of p~ ll.yl~"c, and from about 0 to 58% by weight of pcl~,.. v~l~,~.e. Preferably, the film comprises from about 42 to about 73%
of EVA copolymer having a ~ L...,l;.le content of about 28%, about 27 to about 58% of p~ h,..c, and/or about 27 to 58% of PUI~ u~lu~
It should be ~I,u,u~c~ t~d that an EVA copolymer having a ~h~Jla~,~,lal~ content of about 12 to about 28% by weight can be used alone.
However, because of the present high cost of EVA copolymer, it is preferable that it be in admixture with other cheaper polymers such as pc,l~ll.yl~,..c and pvlyl!l v,uylu,lc, in order to produce consumer goods having a competitive price.
Currently, the processing of PVC material into a marketable item such as, for example, a protective covering for ring binders or wallets for carrying cheque books and other r ' cd goods involves the extrusion of PVC into a film having the desired thickness. The PVC film is then processed in a machine 30 adapted to weld at least two layers of said film around an object to be protected such as cardboard, or to weld at least two layers of said film for making other types of ",~ .Lclh. cv goods like garbage bags, food storage bags and the like, the layers being then welded by high frequency, which also preferably involves a ~
.. .. ...... .. ... .. . .....

WO 92/16358 = PCT/CA92/00125 210~8~

cutting step to discard unwanted material. Heat welding can also be used insteadof high frequency welding. However, from a commercial and industrial point of view, high frequency welding is much faster and efficient.
A, .. ' apparatus for carrying out the method of high frequency welding of the present invention comprises at least a press and means for welding at high frequency. Many types of high frequency welding apparatus are available on the market, such as COLPlTr, KIEE~EL, COSMOS, HIsEN and the like. It should therefore be kept in mind that the method of the present invention 10 can be performed on any of the these machines.
In more details, the method of the present invention compriseS
applying pressure onto at least two layers of an EVA film deflned above by using a press preferably equipped with a heatable plate. The tc.~ a~UI c of the plate can 15 vary from about room tclll~,lal~lc to as high as possible, as long as the film does not start to melt, thus causing it to stick to the plate. In other words, the highest possible 1...,.~.~,. alulc of the plate is the melting point of the ~ of the film to be welded. Preferably the plate is heated around 70-80C because it allows a shorter welding time, and requires less energy for welding.
The pressure applied is function of the thickness of the film, or the number of layers of said film. It is highly desirable to apply such pressure in order to obtain effective welding. Usually, the operator of the high frequency weldingapparatus is well aware of the optimal pressure to be applied.
Some, but not all, high frequency welding apparatus are equipped with a pre-welding time option which permits an improved settlement of the film to be welded, as well as the application of a more uniform pressure on the film, prior to high frequency welding. Though not essential, such pre-welding operation is 30 IC I ~ to obtain better results.
~ul -~ y~ the layers are welded by high frequency at a wayelength of about 27 MHz, which is the usual wavelength provided in high frequency welding W09~/16358 2 1 0 4 8 6 4 ~ PCT/CA92/00125 apparatus. The welding time is also function of numerous ~ ,t~ , such as the type of press (hydraulic, air etc...), the thickness of the layers welded, the l~",u~ lu~ ~ of the plate etc... Again, the operator of the high frequency apparatus is well aware of these ~ a, and can determine the optimal conditions to S maximize the method resulting in a satisfactory weld.
The high frequency welding step is preferably followed by a cooling period in order to allow the newly welded layers to cool down, and the weld to strengthen before the press is removed.
Finally, the excess of material, if any, is stripped, providing the funished goods The high frequency welding step described above can also be replaced 15 by an ultrasonic vibration welding step. As an example of a suitable ultrasonic vibrations welding apparatus, there may be mentioned the Mach Sonic type SWP-14A. Obviously, other ~U~...,llliUI~I ultrasonic vibrations welding apparatus may be used, such as King Ultrasonic Mach. etc .
The user is usually equipped with the machine that serves his purposes, which can be other than welding the ~Il.,,ll,~,~L.al;. films disclosed in the present arrlin~tinn It should also be mentioned that the ultrasonic vibrations welding step is performed at room t~
In developing a th~ lO~)l,.alic substitute for PVC, two critical issues had to be considered. First, the substitute must be non-toxic for the ~ t, with mechanical properties at least similar or superior to current PVC coverings;
and second, it is obviously imperative that the covering material be weldable by high frequency in similar conditions.
The ll,. ,....~,l-~l;, substitute film should also possess the following highly desirable ull~ iali~.

Wo 92/16358 ~= PCT/CA92/00125 2104~6~ 8 ~
- the stripping: the ~ iu..~l method for high frequency welding involves the ' rcuttingr of the excess of material by a cutting dye present in the same matrix. The excess of material is then ~strippedr manually without tearing;

- the rigidity: the film should have a good resistance to torsion and shearing;
- cold crack: since the goods containing a protective covering may be submitted to low t. ",~ u~ , for example, a ring binder forgotten in a car in the winter time, the covering should possess propenies such that it wiU remain flexible even after being submitted to low ~G~ . ru~lh~ lG, the weld should not be affected or altered;
- fatigue: items such as ring binders containing a covering or wallets for cheque books, are opened and closed hundreds of times. It is therefore essential that the covering material and the weld be rGsistant to cracks caused by fatigue.
The ll - " ~ ;, EVA film used in the method of the present invention complies with all the above IG tUiU~ ,..L~, and in some instances, hast~n~ superior propérties and advantages over PVC. Extrusion of the ;.... into a film is carried in a . u" .. ' manner with standard equipment well known in this field of the art. The thickness of the resulting sheGet or film can 25 be adjusted at wiU, depending on its intended use and the limits of the extruding machine. Typically, it varies from about 0.008 to about 0.022 inches.
Many techniques other than extrusion are available to the skiUed workman for obtaining an EVA film suitable for the method of the present 30 invention. However, the extrusion is preferred.
The chemical properties of the EVA fùm and its ~ ;r~-~ products resulting from ~liol~ have been analyzed. Also, the m~rh~nir71 properties of wo 92/16358 2 1~:14 8 6 ~ pcr/cAs2/oo~2s 9, consumers' goods containing at least t~YO layers of an EVA film welded in accordance with the method of the present invention, for example, a ring binder,have been extensively evaluated.
S Upon analysis, it has been found that the EVA layer or film used in the method of the present invention is free of heavy metals such as Ba, Cd, Cr, Se and Hg, and also free of halogen atoms, whereas some PVC samples show up to 2450 ppm of Ba, 1240 ppm of Ba, and 480 ppm Cr. r. ~ c~ ~ of the EVA film leads to water and CO2, which are obviously non-toxic products.
PVC materials contain phthalates used as plasticizers for enhancing the cold crack resistance of the covering. These chemicals are highly toxic for humans and the ellV;lUI~II..,III, and their ' ' may give .1:1...,....I;..,;--c and d;b~ lr~ .S, well known for their high leYel of toxicity.
The cold crack tc~ L.e of PVC is about -20C, whereas the cold crack l~,...t,.-.- c of a protective ring binder covering prepared with the EVA film is lower than -55C.
A fatigue test has been carried out in the following manner: a ring binder protected with a covering consisting of two layers of the EVA film welded by high frequenc,Y with a cardboard inserted in between, was placed on an apparatusallowing opening and closing of said binder. A similar experiment was carried out with a PVC covered ring binder. After 2500 cycles, the PVC binder was severely damaged, whereas the other binder covered with the EVA film remained intact after 80 000 (eighty thousand) cydes.
The EVA film may also contain a colour ~u"~ te, such as, for example, blue ~ c #4PPP201 manufactured and sold by Resco Colors Ltd.
Any other suitable colour c~.,.~,.lll.~le can be used in said film, as long as the high frequency welding properties are not impaired.

21~486~ lo Other additives such as a slip used to lower the coefficient of friction, a filler used to enhance the rigidiy of the film, an anti-block agent used to lower the interfacial blocking properties and the like, can also be added in the film. An example of a slip is Erucamide, r ed and sold by Canada Colors. An 5 example of a filler is talc ' ~;~ and sold by Canada Colors. An example of an anti-block agent is silica, r ' ~d and sold by Canada Colors.
The of each of these additives is ~' ' ' by the intended use of the finished goods. It must be kept in mind, however, that the 10 ~ , aliull should neither alter the weldability of the EVA film, nor the resistance of the weld between two or more layers of the film.
The following examples are provided to illustrate the present invention rather than limit its scope.

EX~MpLF, I
The following products are blended together:
- 68% by weight of ~lh~ cr;.l ~la.,.,ldl'~ copolymer (EVA), #2803M,0 , - . r 1, ~;d and sold by At Plastics, wherein the content of VIIIJ' ' ' is 28%;
- 28% by weight of LLDPE #IIDI, lllallura~lu.~J and sold by DuPont Canada; and - 4% by weight of colour cull~llllate #4PPP201, r ' ~;~ and sold by Resco Colors Ltd., having a pul~,u,u/l~,,.e hu~ùpul~ content of about 25 60% by weight.
This blend is melted, giving a ~ containing 19% by weight of ~iul~la~ lale~ The melted ~ . . is then extruded in a ~u~ ltiùll~l manner with standard extruding equipment to produce a film having a thickness of ~I,U,UlUAilll l~,lr 0.008 inches. Depending on ~he intended use, the thickness can be 30 adjusted as desired.

WO 92/16358 210 ~ ~/CA92~00l25 F.X~MPI.F. 2 Hieh Frequency Wf~l~lin~
A cardboard is placed between two layers of the film prepared in S Example 1, and processed through a high frequency welding machine COLPlTr Serial Number 902216, under the following ~ i conditions:
- I . ' G of the plate: 75C
- pressure applied: 40 bar ,11.. a~ 'y 27 M~z - amplitude: 95 pre-welding time: 05 second - welding: 2.0 second - cooling time: 0.2 second - anode current: between about I to about 2 amperes - grid current: about 05 ampere to lead to the desired binder after the mounting of suitable rings.
The amplitude is the last parameter adjusted on the high frequency welding apparatus. This feature is not present in every apparatus, but when it is, it 20 allows a better fine tuning.
It is to be noted that the anode current and the grid current indicate whether the tuning of the apparatus is adequate. In other words, once all the other are adjusted, the anode current should be between I and 2 amperes, and 25 the grid current should be around 0.5 in order to obtain ~ti~ra~,lu. y high frequency welding of the EVA flm.
ExAMpLF~ 3 Proceeding in the same manner as in Example 1, the following were prepared and extruded to provide a flm. The values listed in the table are in % by weight. For films 4 to 44, the EVA used had a ~ ,late content of 28% by weight.

WO 92/16358 PCT/CA92/00~25 2104~6~ 12 FILM ev~ LLDP LDPE HDPE PPI PP2 PP3 PP~ V
# E

4 42.86 53.14 4 12 5 42.86 53.14 4 12 6 42.86 53.14 4 12 10 7 42.86 53.14 4 12 8 42.86 53.14 4 12 9 42.86 53.14 4 12 . .

WO 92/16358 210 4 8 6 '1 PCI-~CA92/0012~

33 71.5 24.5 4 20 34 71.5 24.5 4 20 35 71.5 24.5 4 20 15 36 715 24.5 4 20 38 71.5 24.5 4 20 39 72.7 23~ 4 æ
40 72.7 23.3 4 22 20 41 72.7 23.3 4 22 42 72.7 23.3 4 æ
43 72.7 23.3 4 22 44 72.7 233 4 22 WO 92/16358 ~~~ PCT/CA92/00125

2~ 0486~ 14 note: PPI = yv4ylvyylu~lc ~v~vyvl~
PP2 = pv:~, uy~lu.~c copolymer PP3 = pv4~uy~1~ ,.c terpolymer PP4 = colour, .lle ~o VA - % of ~ ~' in the film.
F~Mpl F 4 Proceedirlg in the same manner as in Example 1, a film having the following ~ was prepared:
- 68% by weight of cll.~ L..,e~ e copolymer (EVA), #2803M, cd and sold by Al Plas~ics, wherein the content of vinylacetate is 28%;
- 4% by weight of colour ~,u~ le #4PPP201, manufactured and sold by Resco Colors Ltd., having a pol~,uy~lu~ ho".uyvl~ . content of about 15 60% by weight;
- 2.5% of slip master batch which is a 5% by weight dispersion of erucamide in an .:yylvyli~te carrier; and - 255% of a mixture comprising 40% by weight of talc and 60% by weight of LLDPE.
This film provided excellent results under high frequency welding and ultrasonic vibrations welding m accordance with the present invention.
F'f~MPLE 5 Two layers of films 28 to 31 as listed in Example 3, each layer having a thickness of about 0.013 inch, are processed through an ultrasonic vibrations welding machine Mach Sonic type SWP-14A at a 15KHz frequency under the following conditions:

WO 92/163S8 21~ ~ 8 6 ~ pcr~cAs2~oo~5 Film Welding Power Welding Hold Pl P2 delay delay time (sec.) (sec.) (sec.) (sec.) (kg/cm2) (kg/cm2) 28 0.3 0.35 0.25 0.5 2 5 5290.3 035 0.30 05 2 5 30 0.3 0.35 0.35 0.5 2 5 31 0.3 0.35 0.40 0.5 2 5 P~ ~,UIIC~,UUII~.b to the pressure applied initially by the horn on the layers at tirne = 0, and P2 is the pressure applied during the welding.
AI~ALYSIS OF THE F~T M~
Each film obtained in Example 3 was submitted to eYaluation of several mechanical properties that are:
- easiness of welding by high frequency and ultrasonic vibrations - stripping - rigidity - cold crack - weld resistance - fatigue resistance It is to be understood that the films were tested especially for protective covering purposes of consumer goods, and therefore, evaluated as such.
Accordingly, the criteria of evaluation might be different depending on the intended use, for example, a food storage bag or a garbage bag.

WO 92/16358 ~ PCr/CA92/00125 21~4~6~ 16 In order to determine a range of a~,~al~vilily, films #1 and 3 consisted in an EVA copolymer alone with a ~u..c~"~, ~,liu., of \,;..~ ' of 10 and 40% by weight IG~,U.~
S The film #1 cannot be welded by high frequency or ultrasonic vibrations, and therefore, no further evaluation of this film was carried out.
The, ~ of film #3 cannot be extruded at alL Accordingly, the first results indicated that the ~ iV~ of ~;..yh~ c in the film must be l0 higher than 10% and lower than 40% by weight.
The film #2 gave excellent results in all the tests, except for the rigidity, which was rather poor for the intended use, that is protective covering.
In a general manner, films 2 and 4 to 44 possess ,;b lir~ ly higher fatigue resistance over similar PVC sheets, as well as a much lower cold crack c.
The rigidity of the film slightly decreases with the increase of the I,vll~,lllativll of ~ yl~ dlC in the film. On the other hand, the easiness of welding, either by high frequency or ultrasonic vibrations, the stripping and the weld resistance increase S b" r~ Lly with the increase of ~u~ liol~ of ~ h ~ c.
n~ rin~ all the desired .1.~....ct~, ;,li~, films 27 to 34 and the film 25 of Example 1, wherein the l,VllC~,.lll~tiul~ of ~;...rldC.,l~lte in the film is from 18 to 20%, have given the more preferred film for performing the method of the presentinv~ntion.

Claims

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

1. A method of high frequency welding together layers of a thermoplastic film comprising vinylacetate in a concentration of about 12 to 28 % by weight of said thermoplastic film in a copolymer-polymer mixture selected from the group consisting of:
an ethylenevinylacetate (EVA) copolymer and polypropylene, an EVA copolymer and polyethylene, and an EVA copolymer and polypropylene and polyethylene, the method comprising the steps of:
a) pressing together layers of said thermoplastic film between solid surfaces;
b) applying a high frequency electromagnetic wave energy to said pressed layers so as to effect high frequency welding together of said layers, said film being capable of being welded by high frequency wave-energy without requiring the application of heat to said layers of said film prior to high frequency welding said layers; and c) releasing said pressure on said layers, cooling said layers and recovering said welded layers.

2. The method of claim 1 wherein the concentration of vinylacetate in said thermoplastic film is from 14 to 25% by weight of said thermoplastic film.

3. The method of claim 1 wherein the concentration of vinylacetate in said thermoplastic film is about 19% by weight of said thermoplastic film.

4. The method of claim 1 wherein said thermoplastic film comprises:
42 to 100% by weight of an EVA copolymer comprising about 28% by weight of vinylacetate, 0 to 58% by weight of polyethylene, and optionally 0 to 58% by weight of polypropylene.

5. The method of claim 1 wherein said thermoplastic film comprises:
42 to 73% by weight of an EVA copolymer comprising about 28% by weight of vinylacetate, 27 to 58% by weight of polyethylene, and optionally 27 to 58% by weight of polypropylene.

6. The method of claim 1 wherein said method comprising the additional step of warming said solid surfaces in order to accelerate the time required for high frequency welding of said layers.

7. The method of claim 5 wherein said thermoplastic film comprises one or more additives selected from the group consisting of a slip agent, an anti-block agent, and a filler.

8. The method of claim 5 wherein said thermoplastic film has a thickness of about 0.008 to 0.022 inches.

9. The method of claim 5 wherein said high frequency wave energy is applied at a frequency of about 27 Mhz.