Note: Descriptions are shown in the official language in which they were submitted.
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PROTECTIVE POLYMERIC FILM FOR LAMINATION TO METAL
The present invention relates to a protective polymeric film which may be readily laminated to
metal substrates to provide prole~;lion from scratches, solvents, and corrosion, and which exhibits
good sealability and co",pressi~ility when the laminate is used to form metal containers.
Protective polymeric films or coatings are known in the art which can be laminated to metals
such as tin-free steel, tin-plated steel, and aluminum, and then formed into parts such as aerosol
valve mounting cups, aerosol can domes or bottoms, paint cans, foo.d and beverage containers, and
the like. Such films or coatings are used to protect the underlying metal surface from corrosion as
well as to provide r~:si~ldnce to solvents and abrasions. For exdr"p'c, U.S. Patent No. 4,626,157 to
Franek et al, describes a method of making metal containers such as aerosol dispensing conl~i"er~
having top can end members and valve cup members formed from a metal laminate comprising a thin
polymeric layer such as polyester or polypropylene overlaid on the metallic substrate.
I lo-Nevcr, while such films provide resistance to corrosion, solvents and abrasions, due to the
high modulus of the polypropylene film, the metal/film laminates using polypropylene do not exhibit
good sealability and col"l,r ss;b.' ~y when used to fabricate metal containers such as aerosol cans.
Specifically, such films do not fomm a good seal for compression fommed joints or seals between two
pieces of a metal la",i"ate. Such a poor seal may result in leakage of pressurized propellant from a
metal aerosol container formed from the metal laminate.
Further, polypropylene has been found to be difficult to adhere to various metallic sui ~ les.
2 0 Many COUII "er,ial polypropylene film lar";, lales currently in use are adhered tometal with solvent
based-adhesives such as urethane adhesives. However, such adl ,esivcs are undesirable as they are
env"ur""enldlly u"~,iendly and leave the potential for residual solvents from the adhesive to migrate
gradually through the film laminate and contaminate the contents of the metal container. Additionally,
in~de~u~te curing of the adhesive can result in loss of adhesion of the film laminate, particularly
~c"~w.;"g exposure to solvents that may be contained within the ~dbricdled structure. As a result,
polymer films have more recently been laminated to metal using li,er",oplastic adhesive resins. For
example, U.S. Patent No. 4,98û,21 û to Heyes et al describes a two-layer film bonded to the surface of
a metal sheet co",p,i~i. ,9 an inner adhesive layer of an acid-modified polyolefin resin and an outer
layer of a polyolefin.
3 o In an attempt to solve the sealability and co"~,ur~ssibility p-.b!~ lls ~soci~t~d with prior
polypropylene adhesive films, U.S. Patent No. 5,û06,383 to Achille et al teach a blend of
polypropylene and very low density polyethylene and an adhesive layer co",~Jri~i"g a nubber and polar
co",ono",er ",o~ ied a-olefin polymer resin. I lowe~cr, while such films exhibit good coill~ ssiiJilily
t and formability, the films may not provide sufficient r~sisldnce to abrasions and sc,dtches. For
3 5 exdr" !e, during handling, bulk packaging, and shipping of aerosol valve mounting cups, such film
laminates can possibly be damaged by cutting, abrading or puncturing as the metal cups vibrate or
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move against each other. Damage can also occur during part die stamping or forming if the stamping
dies or assembly mechanisms are not perfectly aligned. If the protective polymeric film becomes
damaged, the corrosion resistance of the aerosol valve mounting cups is also weakened.
Accordingly, there is still a need in the art for a protective film which may be readily laminated
5 to a metal substrate without the use of a solvent-based adhesive, which exhibits good se~l~hility and
compressibility, and which is scratch resistant as well as resistant to solvents and corrosion.
The present invention meets that need by providing a protective polymeric film which may be
readily laminated to a metal substrate, which is resistant to scratches, solvents and corrosion, and
10 which exhibits good sealability and corll,ur~ssibility when used to form metal containers. The film may
be produced by conventional coextrusion techr, ,_es
According to one aspect of the present invention, a protective polymeric film is provided
comprising an adhesive layer comprising from 5 to 25 percent of the total film thickness, a scratch-
resistant layer coll lul i~i, lg from 5 to 40 percent of the total film thickness, and a com,ur. ~ ~!t ~e layer
15 positioned between the adhesive and scratch-resistant layers comprising from 40-80 percent of the
total film l h 'c. less. The scratch-resistant layer preferably has an ASTM D2240 Shore D hardness of
at least 60, and the adhesive and co,ll,ur. - ' !e layers have a Shore D hardness which is less than
that of the scratch-r~s;;,ldnl layer. Additionally, it is desired that the scratch-~esislar)l layer have an
ASTM D3363 pencil hardness of at least 3B, and preferably at least B.
2 0 Preferably, the adhesive layer is selected from the group consiilil lg of ethylEn:c - lly
unsaturated copolymers of vinyl acetate, ethyl acrylate, ethyl methacrylate, methyl acrylic acid, acrylic
acid, or carbon monoxide; hon lopolymers of ethylene or propylene; copolymers of ethylene and
propylene; ionomers of ethylene and methacrylic acid or acrylic acid; maleic anhydride modified
polyethylene; polyamides; polyurethanes; or comr ' !e blends thereof. The adhesive layer is
preferably thermally activated for lamination to a metal substrate and is solvent free.
The scratch-resistant layer is preferably select~d from the group consi~lillg of polypropylene,
polyethylene, polyester, polyamide, and blends thereof. Once the protective film is ~ n l! laled to a
metal substrate, the hard scratch resistant layer functions to protect the underlying metal from
scldlches, and provides resistance to solvents and conusion. Optionally, the scratch-resistant layer
3 0 may be coated with curable hard coatings sele-,t~d from the group consisli~ lg of urethane, epoxy,
acrylic and silicone to provide enhanced scratch l~siilance properties.
The scratch-resistant layer may also contain an inorganic filler to further harden the layer and
enhance scratch r~si;il~nce. Chemical or radiation-induced clussli,lhillg may also be used to further
enhance the hardness and scratch resistance prupellies of the scratch-r~ nl layer.
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The cor"p~ ' !e layer preferably comprises homopolymers of ethylene or propylene,
copolymers of ethylene and propylene, ethylene-propylene rubbers, thermoplastic olefinic elastomer
resins, styrene block copolymers, polyurethane, and blends thereof. The co" ,pr~s~ ' !e layer is a
softer polymeric film layer which provides good sealability and coi "~)ressibility properties when the
5 protective film is laminated to a metal substrate and used to fabricate metal containers. In a pr~fér,ed
embodiment of the invention, the co-",oressiL)le layer is foamed using a chemical blowing agent to
~ provide improved compressibility.
~,
The protective polymeric film of the present invention preferably has a thickness of from 0.5 to
20.0 mils (0.013 to 0.51 mm). In a preferred embodiment of theinvention, the protective polymeric
10 film is bonded to at least one major surface of a metal substrate to form a metal la~,i"ate where the
film comprises scratch-resistant, adhesive, and compressible layers as described above. Suitable
metal substrates range from 5 to 100 mils (0.13 to 2.5 mm) in thickness and preferably from 5 to 15
mils (0.13 to 0.38) in thickness. The protective polymeric film is preferably laminated to the metal
laminate by thermally activating the adhesive layer and bonding the adhesive layer to the metal
15 laminate. The film should exhibit at least 5.0 pounds per linear inch (.89 kg/cm) peel adhesion to the
metal when s~e~ ~d to a 180 degree peel test in accordance with ASTM D-903. Thus, the scratch-
resistant layer becomes the exl ~osed outer protective layer of the metal laminate, and the softer,
compressible layer remains between the adhesive and scratch-resistant layers. The protective film
protects the underlying metal substrate from scratches which could expose the metal to solvents and
2 o corrosion.
In an alternative er"bod "enl of the invention, a protective polymeric film is provided which
includes a scratch resistant layer and an adjacent layer which possesses both co",~.c ' '- and
adhesive properties. The compressible adhesive layer comprises from 10 to 90 percent of the total
film thickness and the scratch-resistant layer comprises from 10 to 90 percent of the total film
2 5 ll,: ~. ,ess. The scratch-resistant layer has a Shore D hardness of at least 60, and the co"),or~s ~ !e
adhesive layer has a Shore D hardness which is less than that of the scratch-res;;,larll layer.
Additionally, it is desired that the scratch-resi~ldnl layer have an ASTM D3363 pencil hardness of at
least 3B, and preférdbly at least B.
The col"~essiL)le adhesive layer is p,e~t:rdbly selected from the group consi;jli"g of
30 ethylenically unsatu,dledcopolymers of vinyl acetate, ethyl acrylate, ethyl Illelhac~ylate, methyl acrylic
acid, acrylic acid, carbon monoxide, homopolymers of ethylene or propylene, copolymers of ethylene
and propylene, iono,ner~ of ethylene and methyl acrylic acid or acrylic acid, grafted anhydride
copolymers, polyamides, polyurethanes, and cor,.r ' !e blends thereof. Preferably, the compressible
adhesive layer is foamed.
The scratch-,e:,isl~nl layer is prererdbly selected from the group consiiling of polypropylene,
polyethylene, polye.,tcr, polyamide, or blends thereof. The scratch resistant layer may be coated with
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curable coatings, ~;~OSC' ,ked, or may include organic fillers as described above to provide enhanced
scratch resistance.
The protective polymeric film of the present invention can be larl,;nated to metals such as tin
free steel, tin-plated steel, and aluminum. In one embodiment of the invention, the protective
5 polymeric film is laminated to tin-plated steel to form an aerosol valve mounting cup. The protective
film of the present invention can also be fabricated into a number of other parts including aerosol can
domes, can bottoms, paint cans, metal trays, pans and the like.
Accordingly, it is a feature of the present invention to provide a protective polymeric film which
may be laminated to a metal substrate. It is a further feature of the invention to provide a protective
10 polymeric film which provides resistance to scratches, solvents and cor,-,sion, and which exhibits
good sealability and compressibility when used in the fabrication of metal containers. These, and
other features and advantages of the present invention, will become apparent from the following
detailed description, the accompanying ~ ;ny~, and the appended claims.
Fig. 1 is a cross-sectional view of the protective polymeric film of the present invention
showing scratch-resi;,l~nl, co~ ' 1-, and adhesive layers;
Fig. 2 is a cross-sectional view of an alternative embodiment of the protective film co" Iprisil ,9
scratch-resistant and co" " re sr ' le adhesive layers;
Fig. 3 is a variation of the embodiment shown in Fig. 1 in which the film has been laminated to
a metal substrate; and
Fig. 4 is a perspective view of an aerosol valve mounting cup which has been formed from
the protective film of the present invention.
The protective polymeric film and film/metal laminate of the present invention provides a
co" ' :. ,alion of properties which has not been achieved by prior art protective films. The present
invention utilizes a soft film layer which exhibits good col"~.ressiLi'ity and sealing properties in
col,lbi"ation with a hard film layer which resists nicks and abrasions which could expose the
underlying metal surface and subject it to corrosion. The film of the present invention may be readily
laminated to a metal substrate by thermally activating a solvent-free adhesive film layer, and the
resulting metal laminate may be formed into metal containers which are protected from sc~lclles,
3 o solvents and cor,-,sion. The bond formed between the adhesive film layer and metal substrate is
strong and durable. Depending on the desired end use, the laminate of the present invention can
resist delamination or debondi"g of the film from the metal substrate after 30 days of immersion in
methylene chloride.
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The protective poiymeric film of the present invention may comprise either a 3-layer film
co",~ i"g adhesive, co,nl,r~ssiL!c, and scratch-resistant layers, or a two-layer film comprising a
scratch-resistant layer and an adjacent layer having both compressible and adhesive properties.
Referring now to Fig.1, one embodiment of the protective polymeric film of the present
5 invention is illustrated. The film includes a scratch-resistant layer 10, an adhesive layer 14, and a
compressible layer 12 positioned between the scratch-,~s;~lar,l layer and the adhesive layer. The
scratch resistant layer exhibits a Shore D durometer hardness of at least 60 as tested according to
ASTM D2240. A pencil scratch ,~si~Llnce of at least a "3B" rating according to ASTM D3363 is also
pr~er ~!e. Films suitable for use as the scratch-resistant layer include polypropylene, polyethylene,
10 polyester, polyamide, and blends thereof having the requisite hardness. Although olefin resins such
as polypropylene and polyethylene are preferred, other suitable resins include polyester and
polyamide or nylon resins. However, it is preferred that the scratch-resistant layer should be
comprised of greater than 80 percent polypropylene to provide the desired scratch resistance
properties.
The scratch-resistant layer 10 may be coated with hard curable coatings such as u,~ll,anes,
epoxy, acrylic or silicones to provide enhanced surface scratch or abrasion resistance. Additives such
as inorganic fillers or che"~- -' or radiation induced crosslinking may also be used to further enhance
the hardness and scratch resistance of the scratch-resistant layer.
The compressible layer 12 may be produced from homopolymers of ethylene or propylene,
2 0 copolymers of ethylene and propylene, ethylene-propylene or other olefinic rubbers. Thermoplastic
eldslu,,,ers such as styrenic block copolymers, polyurethane, or blends of these resins are also
suitable for use in the present invention. The collllJr~ssil,'E layer ispreferably foamed using cher":~ '
blowing agents such as sodium bicarbonate-citric acid blends or azodic, rbonamide. The foamed
col"~,r. ~ ~ ' 'e layer provides improved compressibility over a conventionally extruded solid layer.
Suitable adhesive polymers useful as the adhesive layer 14 include, but are not limited to,
ethyl~nk -~y unsaturated copolymers of vinyl acetate, ethyl acrylate, ethyl ",ell,a~,ylate, methyl acrylic
acid, acrylic acid, and carbon ",ono~ide. Other ~w~ es include hon,opolymers of ethylene or
propylene, copolymers of ethylene and propylene, ionomers of ethylene and methyl acrylic acid or
acrylic acid, grafted anhydride copolymers, polyamides, polyurethanes, and coml,t ' !e blends
3 0 thereof. The adhesive layer should be readily adherable to the co",pr s~ ' !e layer as well as to a
metal substrate.
Generally, the scratch-resistant layer co~ l ises from 5-40 percent of the total film ll !-, -ess,
the com,u,~ssible layer col"prises from 40-80 percent of the total film thickness, and the adhesive
layer cor, ~,urises 5-25 percent of the total film ll h~ess. A more preferred structure for the 3-layer film
3 5 iS a scratch-resistant layer col",urisi"g 20 percent of the total lh- k. ,ess, a col",u,~ssible layer
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comprising 65 percent of the total thickness, and an adhesive layer comprising 15 percent of the total
thickness.
An alternative embodiment of the invention is illustrated in Fig. 2 in which the film comprises a
co" ,,.,r. ~ ~" Ic adhesive layer 16 and a scratch resistant layer 10. In this embodiment, the
5 cor"~,ressiL,le adhesive layer preferably comprises 75 percent of the total film thickness and the
scratch-resistant layer comprises 25 percent of the total film thickness.
The scratch-resistant layer may comprise polypropylene, polyethylene, polyester, polyamide,
and blends thereof, with polypropylene being the most preferred. Suitable resins for use as the
col"~,res= ' le adhesive layer include ethylenically unsaturated copolymers of vinyl acetate, ethyl
10 acrylate, ethyl methacrylate, methyl acrylic acid, acrylic acid, carbon monoxide, homopolymers of
ethylene or propylene, copolymers of ethylene and propylene, iono"~er~ of ethylene and methyl acrylic
acid or acrylic acid, grafted anhydride copolymers, polyamides, polyurethanes, and co",~,dliLle blends
thereof.
The protective polymeric film of the present invention is pre~erdbly produced by a coextrusion
15 process on a conventional cast film line or tubular blown film line. The layers may be coextruded
together, and the resulting protective film may then be ll ,er",ally laminated to a metal substrate using
a hot roll la", ,~l.,r. A metal laminate is shown in Fig. 3 in which adhesive layer 14 is adhered to a
metal substrate 18 with the scratch-resistant layer 10 forming an outer protective layer for the metal.
The metal can be coated on one or both sides with the same or different protective films.
The resulting metal laminate may be used to fabricate a variety of parts such as aerosol valve
mounting cups, aerosol can domes, aerosol can bottoms, paint cans, metal pans or trays, and the
like. Fig. 4 illustrates an aerosol valve mounting cup 20 formed by sla""~;.,g the metal laminate
formed from the protective film of the present invention. The col"pressiL,le layer 12 allows the film to
be readily co"",r~ssed for fabric~li"g such parts, and also provides a good seal, such as during
Clil l l, . 19 operations when the aerosol valve mounting cup is later crimped to a can body.
It should also be appreciated that the protective film may be laminated to both sides of a
metal substrate. For example, the film Ih ~"ess may be from 5-10 mils (0.13 mm - 0.25 mm) on the
inside of an aerosol valve mounting cup, fomming a sealing gasket, and from 1-2 mils (0.025 mm -
0.05 mm) on the outside of the cup, forming a scratch resistant, corrosion resisla,ll coating.
3 o In order that the invention may be more readily understood, reference is made to the following
examples, which are intended to be illustrative of the invention, but are not i~lended to be limiting in
scope.
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Example 1
A three-layer protective polymeric film was produced in accordance with the present invention
on a conventional cast film line. The scratch-resistant layer, which comprised 25 percent of the total
film thickness, was comprised of a polypropylene homopolymer (Himont ProFaxTM PD-064). The
compressible layer comprised 60 percent of the total film thickness and was comprised of a blend of
50 percent polypropylene (Himont ProFaxTM PD-064) and 50 percent ultra low density polyethylene
(Dow Chemical ATTANE(~) 4201). The adhesive layer comprised 15 percent of the total film ll ~ -k"ess
and was comprised of a blend of 60 percent Quantum Chemical PlexarTM 360 maleic anhydride
modified linear low density polyethylene and 40 percent Quantum Chemical PlexarTM 206 maleic
anhydride modified high density polyethylene. Each layer was extruded at temperatures of 400-410~F
(204-210~C) with a die temperature of 410~F (210~C). The coextruded film was quenched on a 100~F
(38~C) casting roll and then wound into a roll. The resulting 8.0 mil (0.2 mm) thick film was then
thermally laminated to a 10.5 mil (0.27 mm) tin-plated steel at 350~F (177~C) using a hot roll
laminator. The film exhibited peel adhesion greaterthan 10.0 pounds per linear inch (1.8 kg/cm)
when su~ : I to 180 degree peel testing using an Instron tensile tester (ASTM D-903). The
resulting laminate was successfully stamped into aerosol valve mounting cups on a mult- ~tion
proy,~ssive die. The film maintained excellent adl,esion to the formed mounting cup.
ExamPle 2
2 0 A three-layer 8.0 mil (0.2 mm) thick coextruded film was produced on conventional cast film
line. The outermost scratch resistant layer which was 25 percent of the total film gauge, was
co",~.,ised of a polypropylene random copolymer (Himont Profax 7531). The co"",r~ssible layer (60
percent of film gauge) was comprised of a blend of 50 percent polypropylene random copolymer
(Himont Profax 7531) and 50 percent ultra low density polyethylene (Dow Chemical ATTANE 4202).
The adhesive layer (15 percent of film gauge) was comprised of a blend of 60 percent Quantum
Chemical Plexar 360 maleic anhydride modified linear low density polyethylene and 40 percent
Quantum Chemical Plexar 206 maleic anhydride modified high density polyethylene. Each layer was
extruded at temperatures of 400-410~F (204-210~C) with a die temperature of 410~F (210~C). The
film was quenched on a 100~F (38~C) casting roll and then wound into a roll. The 8.0 mil (0.2 mm)
film was then thermally laminated to 10.5 mil (0.27 mm) tin plated steel at 350~F (177~C) using a hot
roll laminator. The resulting la",;n~le was successfully stamped into aerosol valve mounting cups on
a m~ tion progressive die.
ExamPle 3
Another 8.0 mil (0.2 mm) three-layer protective polymeric film was prepared as in Example 1.
The scratch-resistant layer comprised 25 percent of the total film thickness and was comprised of a
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polypropylene homopolymer (Himont ProFaxTM PD-064). The compressible layer comprised 60
percent of the total film thickness and was comprised of a very low density metallocene catalyst
ethylene copolymer (Dow AFFINITYq~) PL-1840). The adhesive layer comprised 15 percent of the
total film thickness and was comprised of 100 percent Quantum Chemical PlexarTM 360 maleic
anhydride modified linear low density polyethylene. Eachlayer was coextruded as described in
Example 1 and the resulting film was quenched and thermally laminated to a 10.5 mil (0.27 mm) tin-
plated steel.
Example 4
A three-layer protective polymeric film was prepared as in Example 1. The scratch-resistant
layer (25 percent of total film thickness) was comprised of a polypropylene homopolymer (Himont
ProFaxTM PD-064). The cor"l,r~ssiL le layer (60 percent of total film thickness) was col"prised of a
blend of 50 percent polypropylene (Himont ProFaxTM PD-191) and 50 percent ultra low density
polyethylene (Dow Chemical ATTANE(~ 4201) with a 1.5 phr of a sodium bicarbonate-citric acid
based foam concer,l~te (Henley HydrocerolTM CF-20). The endothermic blowing agent was
activated during extrusion to produce a very fine closed cell foam co, l ,~.l . s e layer. The adhesive
layer (15 percent of total film thickness) was comprised of 100 percent Quantum Chemical PlexarTM
360 maleic anhydride modified linear low density polyethylene. Each layer was extruded as described
in Example 1 and the resulting film was quenched and thermally laminated to a 10.5 mil (0.27 mm)
tin-plated steel.
Example 5
A three-layer protective polymeric film was prepared as in Example 1 co"",ri:,i"g a scratch-
resistant layer (25 percent of total film thickness) of a polypropylene homopolymer (Himont ProFaxTM
PD-064) a compressible layer (60 percent of total film th :k. ,ess) comprising a blend of 65 percent
polypropylene (Himont ProFaxTM PD-064) and 35 percent ethylene-propylene rubber (Exxon
Chemical VistalonTM 3708P). The adhesive layer was comprised of 100 percent Quantum Chemical
PlexarTM 360 maleic anhydride modified linear low density polyethylene. Each layer was extruded as
in Example 1 and the resulting film was quenched and thermally laminated to tin-plated steel.
Example 6
A two-layer 8.0 mil (0.2 mm) thick polymer film was produced on a conventional cast film line.
3 o The scratch-resistant layer comprised 80 percent of the total film thickness and was comprised of a
50 percent polypropylene (ProFaxTM PD-064) and 50 percent ultra low density polyethylene blend
(ATTANE(g) 4201). The adhesive layer comprised 20 percent of the total film ll ! k. ~ess and was
comprised of a blend of 60 percent PlexarTM 360 linear low density polyethylene and 40 percent
PlexarTM 206 high density polyethylene. The resulting film was thermally laminated to tin-plated steel
3 5 and stamped into an aerosol valve mounting cup as described in Example 1.
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ExamPle 7
Polypropylene, polyethylene, Plexar adhesives and blends, including those described in
Examples 1-5, were hot pressed into 0.125 inch (0.32 cm) plaques using a heated platen press. The
polymer plaques were then tested for durometer hardness using a Pacific Transducer Corp. Model
~ 5 409 type D durometer tester according to ASTM DZ240. The results are shown in Table 1:
ExamPle 8 ~=
A pencil hardness test (ASTM D-3363) was used to determine scratch resistance of the
plaques in Example 7. A set of Eberhard Faber Design Drawing 3800 pencils with various lead
hardnesses ranging from 6H (hard) to 6B (soft) were used to quantify hardness or scratch resistance
10 of the various polymer blends given in Example 7. The results are shown below in Table 11.
As can be seen, the plaques containing polypropylene or blends of polypropylene and ultra
low density polyethylene exhibit the highest scratch resistance.
ExamPle 9
In order to compare the relative abuse resistance of a film laminate with a 100 percent
15 polypropylene protective layer with that of a film lar"' ,ale with a protective layer cor"prised of 50
percent polypropylene and 50 percent ultra linear low density polyethylene, aerosol valve mounting
cups produced from the laminates desc,ibed in Example 1 and Example 6, respectively, were
sub, s~ted to a vibratory shaker evaluation. Five aerosol valve mounting cups of each sample were
placed into empty glass 9 oz. bottles which were mounted on a Lab-Line Instruments Multi-Wrist
2 0 Shaker. The five identical metal laminate mounting cups in each of the bottles were sl lb, - : ~ ~ to
vigorous shaking at an instrument setting of 10 (maximum) for five minutes. Cups were then
removed and inspected for damage in the form of nicks, scratches or abrasion on the film laminate
side using a 1 OX optical magnifying lupe. The cups of Example 6 (50 percent polypropylene - 50
percent ultra low density polyethylene top layer) exhibited an average of 22 surface damaging nicks
2 5 on the laminate, while the cups of Example 1 (100 percent polypropylene top layer) exhibited an
average of only 9 surface damaging nicks.
While certain representative e"ll,odi",e"l~ and details have been shown for purposes of
illustrating the invention, it will be apparent to those skilled in the art that various changes in the
1ll~11lods and apparatus disclosed herein may be made without departing from the scope of the
3 o invention, which is defined in the appended claims.
