Tubular packaging material, preferably for use as a sausage casing

Disclosed is a multi-layer tubular packaging material having two or more layers, comprising at least one first layer of a polymer comprising a linear polyamide and at least one second layer of a polymer comprising a linear polyamide containing from about 2 to 40 percent by weight of a hydrophilic substance compatible with the polyamide, e.g., polyvinyl alcohol.

The present invention relates to a tubular packaging material and more 
especially to a multi-layer tube based on polyamides, comprising at least 
two layers and having improved physical properties as compared to prior 
art multi-layer tubular films. The tube is ideally suited for packaging 
purposes, in particular, as a wrinkle-free sausage casing for sausages 
which are subsequently treated in hot water or steam. 
From U.S. Pat. No. 3,762,986, multi-layer flat films are known which are 
based on various polyamides and are made up of structures according to the 
systems A/B/A, A/B, or B/A/B. In this case, the A layers represent 
polyamides having melting points below 200.degree. C., e.g., polyamide 11 
or polyamide 12, whereas the B layers represent polyamides having melting 
points above 200.degree. C., e.g., polyamide 6, polyamide 6,6, or 
polyamide 6,10. 
These composite films exhibit a low permeability to moisture, but their 
permeability to, e.g., oxygen is relatively high. The permeation values 
(in cm.sup.3 /m.sup.2.d. bar) for 40 .mu.m thick films range, e.g., for 
polyamide 12 from about 300 to 380 and for polyamide 11 from about 160 to 
200. For many applications, this permeability to oxygen is still too high 
to prevent adverse effects on the goods packed due to oxidation. Problems 
may also be encountered concerning the adhesion values between the 
individual layers, as the films are produced by coextrusion. 
U.S. Pat. No. 3,093,255 discloses the preparation of mixtures of polyamides 
(e.g., of polyamide 6 or polyamide 6,6) and polyethylene, which may, for 
example, be used for manufacturing films or for blow molding bottles. 
Although these products have a low permeability to gases, measurements 
have shown that the permeability to oxygen still exceeds 500 cm.sup.3 
/m.sup.2.d. bar for 40 .mu.m thick films. As indicated above, these high 
permeability values cannot be tolerated in many applications. This United 
States patent does not mention the manufacture of composite films. It is 
only stated that the bottles produced may be coated with polyvinylidene 
chloride, thus reducing their permeability. However, such coatings must be 
regarded as critical for ecological reasons, since toxic vapors are 
generated when the shaped articles are destroyed in combustion plants. 
In German Offenlegungsschrift No. 25 51 023 a polymer mixture is described 
which is composed of 60 to 90 parts by weight of polyamide, 40 to 10 parts 
by weight of polyethylene, and 1.8 to 8.0 parts by weight of a plastic 
graft copolymer. In this case, it is merely mentioned that mixtures of 
this kind may be used for the manufacture of wire insulations or shaped 
articles. 
The journal "Kunststoffe", Volume 65, Number 3, pages 139 to 143, reports 
investigations conducted on plastic mixtures of polyamide 6 and 
high-pressure polyethylene, but it is not possible to infer from this 
report any suggestion for producing films or even composite films using 
these mixtures. 
Many sausage casings are known which are made of thermoplastic films, of 
cellulose hydrate or of combinations of these materials with reinforcing 
or support materials, for example, paper or fabric. Sausage casings, 
however, must meet strict requirements, in particular relating to their 
physiological suitability, their strength during tying off and filling, 
their form stability during scalding, their softness during processing by 
hand, the wrinkle-free appearance of the finished sausages and low cost. 
None of the materials used for the manufacture of tubular films (e.g., 
polyethylene, polyester, the various polyamides, polyvinylidene chloride 
and polyvinyl acetate) possesses all of the properties required, without 
simultaneously exhibiting certain disadvantages. Polyethylene, e.g., is 
highly suitable for physiological reasons, it is soft in handling and 
extremely cheap; but it has the disadvantages of a low strength and an 
insufficient form stability. 
One drawback of tubular films of many synthetic materials is that they tend 
to become wrinkled when the meat or sausage emulsion packed in these 
materials cools down. This is, above all, the case when the tubular films 
are used as casings for sausages which are, e.g., subjected to a 
sterilizing treatment by subsequently heating them. As is known, sausages 
which are to be treated in hot water or steam must be heated in a scalding 
kettle or in a cooking cabinet after filling. During this treatment, the 
volume of the sausage emulsion increases. As long as heat is supplied, the 
casing material has a temperature of 75.degree. to 80.degree. C. or above, 
and the sausage casing expands together with the sausage emulsion. As the 
sausages cool progressively from the outside to the center, the volume of 
the sausage emulsion decreases again; however, shrinking of the sausage 
casing is insufficient so that the casing only partially reassumes its 
original condition prior to heating. As a result, longitudinal creases are 
formed, which diminish the value of the sausages, as far as their 
appearance is concerned, because the consumer will associate the wrinkled 
condition with age. The reason for the formation of longitudinal creases 
is that the strength of the sausage casing at the scalding temperature, 
and thus its resistance to deformation, is only a fraction of its strength 
at 20.degree. C. In addition, the sausages are abruptly cooled, for 
example, using cold water, after they have been treated at elevated 
temperatures. 
In order to reduce the disadvantages of the prior art sausage casings, 
composite tubes have been proposed having inner layers which are composed 
of materials which are resistant to dilute acids and which are practically 
incapable of absorbing water. See German Offenlegungsschrift No. 23 58 
560. The preferred materials mentioned are longer-chain polyamides, e.g., 
polyamide 11 or 12. The outer layers should consist of a polyamide, e.g., 
polyamide 6 or polyamide 6,6, or polyvinyl acetate, which have a 
comparatively high absorbing power for water. The comparatively high 
absorbing power for water of the outer layers is achieved by subjecting 
these layers to an acid treatment. For this purpose, inorganic and organic 
acids are used in dilute form, with hydrochloric acid being preferred. 
Although these tubes lead to a certain improvement with respect to the 
wrinkle-free appearance of the sausages, as compared to prior art sausage 
casings, the acid treatment is, nevertheless, a disadvantage, since it may 
change the materials. In addition, it cannot be excluded that an acid 
residue remains in the tubes in spite of the washing procedures specified, 
and this is critical for physiological reasons. Furthermore, these tubes 
still do not have optimum values regarding their permeability to gases and 
vapors, in particular oxygen and water vapor, and also, an absolutely 
wrinkle-free appearance of the sausages, as desired by the customer, 
cannot be achieved. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the invention to provide a tubular packaging 
material which has improved barrier properties, in particular with respect 
to oxygen and water vapor, and which, in its preferred application as a 
sausage casing, encloses the sausage emulsion without forming wrinkles 
after the scalding process. 
In accomplishing the foregoing objects, there has been provided in 
accordance with the present invention a multi-layer tubular packaging 
material having two or more layers, comprising at least one first layer of 
a polymer comprising a linear polyamide and at least one second layer of a 
polymer comprising a linear polyamide containing from about 2 to 40 
percent by weight of a hydrophilic substance compatible with the 
polyamide. Preferably, the hydrophilic substance comprises polyvinyl 
alcohol which is obtained by 50 to 100 mol percent saponification of 
polyvinyl acetate and usually shows a degree of polymerization of 500 to 
2000. The method for producing PVA is described, for example, in U.S. Pat. 
No. 3,440,316 and in U.S. Pat. No. 3,409,598, the disclosures of which are 
hereby incorporated by reference. Especially, a partially saponified PVA 
including less than 15 percent of unsaponified ester groups is used. 
According to another preferred aspect, the polyamide containing the 
hydrophilic substance comprises a polyamide having from 4 to 6 carbon 
atoms in each unit, preferably polyamide 6. In one embodiment, the first 
polyamide layer is bonded to the inside of the second polyamide layer 
containing the hydrophilic substance and comprises a polyamide having 9 to 
12 carbon atoms in each unit, preferably polyamide 11 or polyamide 12. In 
another embodiment, the first polyamide layer comprises a polymer blend of 
polyamide and polyethylene. In still another embodiment, the first 
polyamide layer comprises a polymer blend of polyamide and polypropylene, 
and in yet another it comprises a polymer blend of two polyamides and 
polyethylene. According to still another embodiment, the packaging 
material comprises at least one additional third layer of a thermoplastic 
polymer containing no polyamide, preferably a heat-sealable material. 
Other objects, features and advantages of the present invention will become 
readily apparent from the detailed description of preferred embodiments 
which follows, when read in conjunction with the attached figures of 
drawing.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The present invention provides a tubular packaging material comprising at 
least two layers, with at least two of the layers being made up of linear 
polyamides. This tubular packaging material is characterized by the 
feature that at least one layer is composed of a linear polyamide 
containing 2 to 40, preferably 5 to 20 percent by weight of a hydrophilic 
substance compatible with the polyamide. 
A considerable number of substances which are compatible with the polyamide 
may be employed, but it has been found that polyvinyl alcohol is 
especially suitable. In practice, polyvinyl alcohol in a partially 
saponified form containing less than 15 percent of unsaponified ester 
groups has proved particularly useful. 
Although all usual types of polyamides may be used as the polyamides 
containing the hydrophilic substance, those containing 4 to 6 carbon atoms 
in each unit are preferred within the scope of the present invention. Of 
these, polyamide 6 is particularly preferred. 
At a calculated thickness of 40 .mu.m, the multi-layer tubular films 
preferably have a permeability value of &lt;25, in particular &lt;20 cm.sup.3 
/m.sup.2.d. bar for oxygen and &lt;20 g/m.sup.2.d for water vapor, 
respectively. 
The polyamide layer which is bonded to the polyamide layer containing the 
hydrophilic substance is preferably composed of polyamides having 9 to 12 
carbon atoms in each unit, with polyamide 11 or polyamide 12 being 
particularly useful. However, since the last-mentioned polyamides are 
relatively expensive, it is preferred, according to the present invention, 
to substitute these polyamides by polymer blends composed of polyamides 
and polyethylene. Suitable polymer blends are in particular those 
comprising 25 to 75, preferably 40 to 60 percent by weight of polyamide 
and 75 to 25, preferably 60 to 40 percent by weight of polyethylene. As 
far as the properties sought after are concerned, particularly good 
results are achieved when polymer blends of polyamide 6 and, preferably, a 
low-density polyethylene (high-pressure polyethylene) are used. 
In order to obtain particular physical properties, for example, with a view 
toward ultimate tensile strength and/or suitability for printing and/or 
sealability, it is intended, within the scope of the invention, to apply 
additional layers, preferably composed of polyesters, polypropylene or 
sealing layers, for example, based on polyethylene. 
The films are preferably used in a non-oriented form, but they may also be 
oriented in at least one direction to obtain particular physical 
properties, for example, an increased ultimate tensile strength. For this 
purpose, the known stretching methods are employed. The films may also be 
heat set to obtain a heat stable film, and this is also done according to 
known methods. 
The multi-layer tubular films according to the invention are particularly 
suitable for use as sausage casings, where it is important to have a low 
permeability to oxygen and water vapor, and, in addition, a wrinkle-free 
enclosure for the sausage emulsion is desired. 
The films may be produced according to known techniques, for example, by 
melt coating and other processes. Coextrusion is, however, preferred. 
Below, the invention is more fully explained in detail by means of selected 
illustrative examples, however, without limiting it to the embodiments 
described therein. 
EXAMPLE 1 
A two-layer tubular film is prepared by coextrusion through an annular slot 
die. The inner layer has a thickness of 40 .mu.m and is composed of a 
polynmer blend of 50 percent by weight of polyethylene having a density of 
0.922 and 50 percent by weight of polyamide 6 having a viscosity of 255 
ml/g and a melting range from 217 to 221.degree. C. The outer layer has a 
thickness of 30 .mu.m and is composed of a polymer blend of 85 percent by 
weight of the above polyamide 6 and 15 percent by weight of a polyvinyl 
alcohol having a viscosity of 1.8.times.10.sup.-2 Pa.s and a degree of 
hydrolysis of 88 percent. 
EXAMPLE 2 
A two-layer tubular film is prepared by coextrusion using the polymer 
blends of Example 1, but in this case both layers have a thickness of 25 
.mu.m. 
EXAMPLE 3 
A two-layer tubular film is prepared by coextrusion. The inner layer has a 
thickness of 35 .mu.m and is composed of a polymer blend of 60 percent by 
weight of polyethylene having a density of 0.918 and 40 percent by weight 
of polyamide 6 having a viscosity of 320 ml/g and a melting range from 217 
to 221.degree. C. The outer layer has a thickness of 35 .mu.m and is 
composed of the polymer blend used as the outer layer in Example 1. 
EXAMPLE 4 
A two-layer tubular film is prepared by coextrusion. The inner layer has a 
thickness of 25 .mu.m and is composed of a polyamide 12 having a viscosity 
of 250 ml/g and a melting range from 176 to 180.degree. C. The outer layer 
has a thickness of 25 .mu.m and is composed of the polymer blend used as 
the outer layer in Example 1. 
EXAMPLE 5 
A 40 .mu.m thick tubular film is prepared, which is composed of a polymer 
blend of 70 percent by weight of the polyethylene mentioned in Example 1 
and 30 percent by weight of the polyamide 6 mentioned in Example 1. After 
this tubular film has left the annular slot die, the polymer blend used as 
the outer layer in Example 1 is melt laminated to it, and for this purpose 
a second, concentrically arranged annular slot die is used. The outer 
layer has a thickness of 40 .mu.m. 
EXAMPLE 6 
A two-layer tubular film is prepared by coextrusion. The inner layer has a 
thickness of 40 .mu.m and is composed of a polymer blend of 40 percent by 
weight of the polyethylene mentioned in Example 1 and 60 percent by weight 
of a polyamide 6 having a viscosity of 250 ml/g and a melting range from 
217 to 221.degree. C. The outer layer has a thickness of 40 .mu.m and is 
composed of the polymer blend used as the outer layer in Example 1. 
EXAMPLE 7 
A two-layer tubular film is prepared by coextrusion. The inner layer has a 
thickness of 40 .mu.m and is composed of a polymer blend of 50 percent by 
weight of a polypropylene having a density of 0.9 and 50 percent by weight 
of a polyamide 6 having a viscosity of 320 ml/g and a melting range from 
217.degree. to 221.degree. C. The outer layer has a thickness of 40 .mu.m 
and is composed of the polymer blend used as the outer layer in Example 1. 
EXAMPLE 8 
A two-layer tubular film is prepared by coextrusion. The inner layer has a 
thickness of 35 .mu.m and is composed of a polymer blend of 42 percent by 
weight of the polyethylene specified in Example 1 and 58 percent by weight 
of a copolyamide 6/6,6 having a viscosity of 240 ml/g and a melting range 
from 214.degree. to 217.degree. C. The outer layer has a thckness of 40 
.mu.m and is composed of the polymer blend used as the outer layer in 
Example 1. 
EXAMPLE 9 
A two-layer tubular film is prepared by coextrusion. The inner layer has a 
thickness of 30 .mu.m and is composed of a polymer blend containing equal 
percentages by weight of the polyethylene and the polyamide specified in 
Example 1 and of a polyamide 12 having a viscosity of 250 ml/g and a 
melting range from 176.degree. to 180.degree. C. The outer layer has a 
thickness of 30 .mu.m and is composed of the polymer blend used as the 
outer layer in Example 1. 
EXAMPLE 10 
A two-layer tubular film is prepared by coextrusion. The inner layer has a 
thickness of 40 .mu.m and is composed of a polymer blend of 40 percent by 
weight of the polyethylene specified in Example 1 and 60 percent by weight 
of the polyamide 6 mentioned in Example 1. The outer layer has a thickness 
of 40 .mu.m and is composed of a polymer blend of 90 percent by weight of 
the polyamide 6 of Example 1 and 10 percent by weight of the polyvinyl 
alcohol of Example 1. 
EXAMPLE 11 
A two-layer tubular film is prepared by coextrusion. The inner layer has a 
thickness of 40 .mu.m and is composed of the polymer blend of Example 10. 
The outer layer has a thickness of 40 .mu.m and is composed of 90 percent 
by weight of the polyamide 6 specified in Example 1 and 10 percent by 
weight of a polyvinyl alcohol having a viscosity of 2.times.10.sup.-2 Pa.s 
and a degree of hydrolysis of 98 percent. 
EXAMPLE 12 
A two-layer tubular film is prepared by coextrusion. The inner layer has a 
thickness of 35 .mu.m and is composed of the polymer blend of Example 10. 
The outer layer also has a thickness of 35 .mu.m and is composed of a 
polymer blend of 96 percent by weight of the polyamide 6 specified in 
Example 1 and 4 percent by weight of the polyvinyl alcohol mentioned in 
Example 1. 
EXAMPLE 13 
A two-layer tubular film is prepared by coextrusion. The inner layer has a 
thickness of 40 .mu.m and is composed of the polymer blend of Example 10. 
The outer layer has a thickness of 40 .mu.m and is composed of a polymer 
blend of 99 percent by weight of the polyamide 6 specified in Example 1 
and 1 percent by weight of the polyvinyl alcohol of Example 1. 
EXAMPLE 14 
A two-layer tubular film is prepared by coextrusion. The inner layer has a 
thickness of 15 .mu.m and is composed of the polyethylene specified in 
Example 1, and the outer layer (thickness 25 .mu.m) is composed of the 
polymer blend used as the inner layer in Example 10. After the coextruded 
tubular film has left the die, a third layer (thickness 35 .mu.m) is 
applied by means of a concentrically arranged die, which third layer is 
made up of the polymer blend used as the outer layer in Example 1. The 
innermost layer of this three-layer tube, which consists of polyethylene, 
produces an extremely good heat-sealability. 
The viscosity measurements for the products mentioned in the examples are 
carried out according to DIN No. 53729 in the case of polyamide 12, and 
according to DIN No. 53727 in the case of polyamide 6 and polyamide 6/6,6. 
The polyvinyl alcohol is measured as a 4 percent strength aqueous solution 
according to DIN No. 53015. 
The polymer blends are prepared using the appropriate known plastification 
equipment, such as disc compressors or, preferably, single or multiple 
screw extruders. 
The adhesion values and the values for the permeation of oxygen and water 
vapor are listed in the table which follows. These values show the good 
barrier properties of the tubes according to the present invention in 
comparison to prior art films. 
The tubular films prepared according to Examples 1 to 13 are used for the 
production of sausages treated in hot water. The table also indicates the 
fit of the sausage casings, i.e., their appearance following cooling. 
TABLE 
______________________________________ 
Permeation calculated 
Appearance 
for a total thickness 
of the 
Ex- of 40 .mu.m sausages 
am- Adhesion H.sub.2 O O.sub.2 following 
ple (N/15 mm) (g/m.sup.2 . d) 
(cm.sup.3 /m.sup.2 . bar . d 
cooling 
______________________________________ 
1 2.8 3 15 +++ 
2 2.8 5 18 +++ 
3 2.0 4 15 +++ 
4 1.0 12 18 + 
5 1.0 3 12 ++ 
6 3.0 6 12 +++ 
7 2.0 3 12 +++ 
8 2.5 3 12 +++ 
9 2.0 10 15 ++ 
10 2.5 5 11 +++ 
11 1.5 5 16 + 
12 2.8 5 11 +++ 
13 2.8 5 16 + 
14 1.0 1 12 
______________________________________ 
The permeation values are calculated for the same total film thickness 
(calculated thickness) to obtain absolutely comparable values. 
The adhesion values are determined according to the socalled TPeel Method 
which is described in "Adhesive Age" , Sept. 1972, page 21. 
+ Satisfactory 
++ Good 
+++ Very Good