Heat-sealable multilayer film having good surface slip with improved thermoformability based on polyamide and polyolefins

Heat-sealable composite film having good surface slip with very good thermoforming properties based on polyamide and polyolefins, wherein the film consists of PA0 at least one layer of aliphatic polyamide (A) and PA0 at least one layer of a polymer blend (B) consisting of 10-60 wt. % of amorphous polyamide and 40-90% of aliphatic polyamide and PA0 at least one thermoplastic heat sealing layer (C) and PA0 at least one bonding layer (D), arranged in such a manner that a heat sealing layer and a layer of aliphatic polyamide are located on the outer sides of the film, and wherein the film is not stretched.

The present invention relates to a multilayer film based on polyamide and 
polyolefins, which is distinguished by particularly good thermoformability 
with simultaneously good surface slip. Such a film is in particular used 
for packaging foodstuffs. 
BACKGROUND OF THE INVENTION 
Combinations of polyamide and polyolefins are traditionally characteristic 
of flexible thermoforming films used for packaging foodstuffs such as 
sausage, cheese etc.. In such applications, polyamide is conventionally 
used as the support material and polyolefins as the material of the heat 
sealable layer (for example GB 2 023 088). The type of polyamide 
traditionally used in thermoforming films is predominantly polyamide 6. 
At variance with the use of pure polyamide 6, the patent literature makes 
reference to the use of polyamide blends prepared from amorphous and 
aliphatic polyamides, for example in stretched films (EP 0 065 278, FR 2 
348 805, U.S. Pat. No. 4,800,129). These films have the disadvantage that, 
due to the stretching (efforts are generally made, for example on grounds 
of strength and/or cost, to achieve the greatest technically feasible 
biaxial or monoaxial drawing), they are no longer thermoformable and are 
therefore fundamentally non-usable for the required applications. 
The use of polyamide blends containing aliphatic and amorphous polyamide in 
thermoformable films is also described. The patent literature makes 
reference to films which manage with a single layer of such a polyamide 
blend (EP 0 408 390, AU 8 825 700, EP 0 358 038, JP 1 006 056, DE 2 309 
420). Such films are characterised in that, while they are indeed more 
readily drawable (for example EP 0 408 390, example III, table 3) than 
polyamide 6 films (polyamide 6 is distinctly more crystalline than a 
polymer blend containing amorphous polyamide), they have distinctly higher 
coefficients of friction (Jacobi, H. R., Kunststoffe 47 (1957); Vieweg, 
R., Muller, A., Kunststoffhandbuch volume IV, C. Hanser Verlag, Munich, 
1966, page 540) and thus have poor surface slip. 
Films are also known from the patent literature which manage with two 
layers, wherein one layer consists of a polyamide blend containing 
aliphatic and amorphous polyamide and another layer consists of 
heat-sealable material (EP 0 526 814, JP 60 097 850, EP 0 287 839, EP 0 
104 436). While, in comparison with the single layer films, these films do 
indeed have the advantage of being heat-sealable, the problem of poor 
surface slip nonetheless remains. 
In brief, it may be stated that the patent literature contains no reference 
to films based on polyamide and polyolefins which simultaneously exhibit 
the properties of good thermoformability, heat-sealability and good 
surface slip. 
The object thus arises of providing a film based on polyamide and 
polyolefins which simultaneously exhibits the properties 
good thermoformability 
heat-sealability and 
good surface slip. 
SUMMARY OF THE INVENTION 
Surprisingly, this object could be achieved by means of a heat-sealable 
composite film having good surface slip with very good thermoforming 
properties based on polyamide and polyolefins, which is characterised in 
that the film consists of at least one layer of aliphatic polyamide (A) 
and at least one layer of a polymer blend (B) consisting of 10-60 wt. % of 
amorphous polyamide and 40-90% of aliphatic polyamide and at least one 
thermoplastic heat sealing layer (C) and at least one bonding layer (D), 
arranged in such a manner that a heat sealing layer and a layer of 
aliphatic polyamide are located on the outer sides of the film. The film 
must not be stretched. 
DETAILED DESCRIPTION 
The polyamide consists of the aliphatic polyamides PA 6, PA 11, PA 12, PA 
66, PA 6,66, PA 6,8, PA 6,9, PA 6,10, PA 6,11, PA 6,12, a copolymer 
prepared from the monomer units contained therein or of a mixture of the 
stated polyamides. 
The amorphous polyamide is a polyamide produced from isophthalic acid 
and/or terephthalic acid with alkyl-substituted hexamethylenediamine. 
The thermoplastic heat sealing layer must have a crystallite melting point 
of 150.degree. C. or below and originate from the group comprising 
polyethylenes, polyethylene copolymers, polypropylene, polypropylene 
copolymers, polybutylenes or ionomers. These are preferably polyethylene 
(LD, LLD), ethylene/vinyl acetate, ethylene/propylene copolymer, Zn or Na 
type ionomer, polyisobutylene, poly-1-butene or ethylene/(meth)acrylic 
acid copolymer. 
The bonding layers consist of an adhesive system and/or a polymeric 
coupling agent. The adhesive system is a 2-component polyurethane adhesive 
system. The polymeric coupling agent is an anhydride-modified 
polyethylene, an acid copolymer of ethylene, an acid-modified ethylene 
vinyl acetate, an acid-modified ethylene (meth)acrylate, 
anhydride-modified ethylene (meth)acrylate, an anhydride-modified ethylene 
vinyl acetate, an acid/acrylate-modified ethylene vinyl acetate or a 
polymer blend containing at least one of the stated coupling agents. The 
coupling agent is preferably an anhydride-modified polyethylene or 
polypropylene copolymer. 
At least one layer may be provided with lubricants and/or anti-blocking 
agents, wherein the lubricant is preferably an amide wax and the 
anti-blocking agent a modified natural silica product. It is particularly 
worthwhile incorporating lubricants and anti-blocking agents into the heat 
sealing layer and/or the aliphatic polyamide layer. 
Preferred film structures are: 
A/D/B/D/C, 
A/B/A/D/C or 
A/EVOH/B/D/C, wherein EVOH is an ethylene/vinyl alcohol copolymer. 
Interlayers may optionally be arranged between layers D and C, which 
interlayers have good adhesion to D and C and may, for example, be 
identical to C. 
Total film thickness is 15 to 400 .mu.m, preferably 50 to 330 .mu.m. 
The film is suitable for printing. At least one layer may be coloured or 
printed. 
The film is in particular suitable for packaging applications, in 
particular for packaging foodstuffs. The film is suitable for packaging 
meat and sausage products, milk products, fish and smoked foodstuffs, 
pre-cooked dishes, bread and bakery goods and medical devices. 
It has surprisingly proved possible by means of the composition of the film 
according to the invention to satisfy the requirement for good surface 
slip while simultaneously achieving good thermoformability. It is known 
from the prior art that, in comparison with aliphatic polyamides, 
polyamide blends containing aromatic polyamide are somewhat more readily 
thermoformable; such blends are, however, characterised by poor surface 
slip (see above, prior art). It could thus be expected in the combination 
according to the invention of both layers that opposing effects would 
result in moderate thermoformability. Surprisingly, however, still better 
thermoformability with good surface slip are achieved. 
Production processes which may be considered for the film are coextrusion 
(blown film or flat film) or also individual production of layers A, B and 
C, which are then laminated together (layer D). Combined processes are 
also conceivable. 
Known, conventional prior art plant designs are used, wherein in the case 
of blown film coextrusion the production process is characterised in that 
the melt is shaped into a film bubble, inflated, cooled and the other, now 
cool, end is flattened by pinch rolls and held closed and the film then 
wound. In the case of flat film coextrusion, so-called chill roll units 
are used, which have the particular feature of large cooling rolls which 
receive the molten film leaving the die. 
The units to be used are fundamentally different from so-called stretching 
units, which cannot be used for the film of the present invention since 
they always produce a stretched product. 
The following combined processes are particularly economically viable: 
flat film coextrusion of a support with the structure A/D/B/D and 
subsequent extrusion or coextrusion coating of layer C, optionally with 
interlayer(s) between D and C. 
flat film coextrusion of support A/EVOH/B, application of an adhesive D and 
subsequent lamination of a heat sealing layer C previously produced as a 
blown film, optionally with interlayer(s) between D and C. 
flat film coextrusion of support A/B/A, application of an adhesive D and 
subsequent lamination of a heat sealing layer C previously produced as a 
blown film, optionally with interlayer(s) between D and C. 
Thermoformability was used as a feature for evaluation of the invention. In 
order to determine thermoformability, the previously produced film samples 
were tested in modem automatic thermoforming machines as are used in the 
packaging industry (for example Tiromat, Multivac). To this end, the film 
webs clamped in the machine were heated in sections by a hot plate. 
Heating may be performed to this end either from the sealing side or also 
from the opposite side to the sealing side. The films preheated at hot 
plate temperatures of 90.degree. C. were then thermoformed into a tray of 
edge dimensions 185.times.115 mm. By increasing the depth of draw in 5 mm 
steps, the maximum depth of draw to which the particular films could be 
thermoformed without defects was determined. 
The frictional behaviour of the films (opposite side to sealing side) 
against metal was also determined. The coefficient of static friction to 
DIN 53 375 was measured. A test apparatus (VNNG) from Otto Brugger, Munich 
was used for this purpose. The measurement conditions were: 
______________________________________ 
test strip: 800 mm .times. 200 mm 
test table: polished steel 
sled: mass 200 g, test surface 63 mm .times. 64 mm 
take-off speed: 
100 mm/min 
test distance: 
&gt;60 mm 
force measurement: 
electronic 
______________________________________ 
Puncture tests to DIN 53 373 were also performed. To this end, film samples 
of a diameter of 80 mm were cut from the finished film webs with a circle 
cutter. The puncture test was performed at an impact velocity of 4.5 m/sec 
with a Dynatester. The direction of puncture is here perpendicular to the 
surface of the clamped sample. Table 1 shows the puncture force in N! 
determined using this method. 
The elongation at break values shown in table 1 were measured by tensile 
testing. To this end, specimens of a width of 15 mm (clamping distance 100 
mm, test speed 100 mm/min) were subjected to tensile force until break. 
Elongation at break is then the elongation of the specimens at break in 
%!. A computer-controlled tensile tester was used.

EXAMPLES & COMATIVE EXAMPLES 
The following examples are intended to illustrate the subject matter of the 
invention. Stretched films have not been examined since it is well-known 
to those skilled in the art that such films are not thermoformable. 
A. EXAMPLE 1 
Multilayer non-stretched film with the structure 
##EQU1## 
The five-layer film was coextruded as a flat film. Total thickness is 145 
.mu.m. The PA 6 was a polyamide 6 of a density of 1140 kg/m.sup.3 with a 
crystallite melting point of 219.degree. C. and a relative solution 
viscosity of 3.8 (PA concentration 1%, temperature 25.degree. C., measured 
in m-cresol), the aPA used was an amorphous polyamide based on isophthalic 
acid and terephthalic acid of a density of 1190 kg/m.sup.3 and a glass 
transition temperature of 127.degree. C., the HV used was a maleic 
anhydride grafted linear low density polyethylene of a density of 910 
kg/m.sup.3 with a crystallite melting point of 125.degree. C. and a melt 
flow index (MFI 190/2.16) of 4.0 g/10 min and the PE used was a copolymer 
of ethylene and octene (LLDPE) of a density of 935 kg/m.sup.3 and a 
crystallite melting point of 126.degree. C. and a melt flow index (MFI 
190/2.16) of 4.4 g/10 min. 
B. EXAMPLE 2 
Multilayer non-stretched film with the structure 
##EQU2## 
Production and polymers as in example 1. 
C. EXAMPLE 3 
Multilayer non-stretched film with the structure 
______________________________________ 
PA 6/HV/(85% PA 6 + 15% aPA)/HV/PE 
35/10/35/10/55 .mu.m 
______________________________________ 
Production and polymers as in example 1. 
D. COMATIVE EXAMPLE 1 
Multilayer non-stretched film with the structure 
______________________________________ 
PA 6/HV/PA 6/HV/PE 
35/10/35/10/55 .mu.m 
______________________________________ 
Production and polymers as in example 1. 
E. COMATIVE EXAMPLE 2 
Multilayer non-stretched film with the structure 
______________________________________ 
PA 6/HV/(20% PA 6 + 80% aPA)/HV/PE 
25/10/25/10/55 .mu.m 
______________________________________ 
Production and polymers as in example 1. 
F. COMATIVE EXAMPLE 3 
Multilayer non-stretched film with the structure 
______________________________________ 
(85% PA 6 + 15% aPA)/HV/(85% PA 6 + 15% aPA)/HV/PE 
35/10/35/10/55 .mu.m 
______________________________________ 
Production and polymers as in example 1. 
Table 1 shows the thermoformability rating (by means of maximum depth of 
draw and elongation at break), frictional behaviour (by means of 
coefficient of static friction) and mechanical strength (by means of 
puncture force) of the described films A, B, C, D, E and F. Testing was 
performed as described above. 
TABLE 1 
__________________________________________________________________________ 
Frictional behaviour, thermoformability and mechanical strength. 
Coefficient of 
Elongation at 
Maximum depth 
Puncture 
static friction 
break of draw force 
Film structure 
--! %! mm! N! 
__________________________________________________________________________ 
A Example 1 
according to 
0.13 626 85 280 
the invention 
B Example 2 
according to 
0.16 586 85 300 
the invention 
C Example 3 
according to 
0.16 689 80 280 
the invention 
D Comparative 
polymer blend 
0.14 447 70 260 
Example 1 
absent 
E Comparative 
excessive aPA 
0.16 461 65 200 
Example 2 
in polymer 
blend 
F Comparative 
PA 6 layer 
0.52 677 85 260 
Example 3 
absent 
__________________________________________________________________________ 
As may be seen from table 1, good coefficients of friction (distinctly 
&lt;0.20) with simultaneously good thermoformability (in this case maximum 
depth of draw .gtoreq.80 mm, elongation at break &gt;580%) are achieved as 
desired in films A, B and C produced according to the invention. The films 
produced according to the invention moreover exhibit elevated mechanical 
strength, as is shown by the puncture force measurement (.gtoreq.280N). 
In film D, which was not produced according to the invention, the polyamide 
blend layer was omitted and, while good frictional values are achieved, 
the film has poor thermoforming properties (in this case maximum depth of 
draw .ltoreq.70 mm, elongation at break .ltoreq.470%) and has low 
mechanical strength (puncture force .ltoreq.260N). A similar assessment 
applies to film E, which has an excessively high proportion of amorphous 
polyamide in the polyamide blend. The outer PA 6 layer is absent in film 
F. While adequate thermoformability is indeed achieved in this case, the 
film is very dull (coefficient of static friction 0.52) and is of low 
mechanical strength.