Polyurethanes and food casing made therefrom particularly by extrusion and stretching

A novel elastic polyurethane having outstanding properties for use in making mono- and multi-layer food casings, more especially sausage casings, by extrusion. Stretching, especially biaxial stretching, gives further improvement. The casings can be boiled and scalded without folding or wrinkling when cooled. The polyurethane comprises a particular combination of components which imparts the desired properties.

This invention relates to elastic polyurethanes, to food casings having at 
least one layer made from the polyurethanes, more especially sausage 
casings, and to processes for the manufacture of such casings. 
Food casings according to the invention are especially suitable for boiling 
and scalding sausages which, after filling, are heated in ater or steam to 
temperatures of approximately 70.degree. to 100.degree. C. The following 
problem occurs with these sausage goods: on heating, which is usually 
carried out in a boiling or scalding vessel, the volume of the sausage 
material increases and the casing material expands with it. Although the 
sausages are initially taut and practically wrinkle-free and are therefore 
of satisfactory appearance, when the sausage cools down after scalding or 
boiling, occurring from the outside inwards, the volume of the sausage 
material is reduced again and the sausage casing shrinks -- but only 
partly to the state it was in before heating. As a result the sausage has 
a wrinkled and unpleasant appearance. The consumer, however, equates a 
wrinkled and unpleasant appearance with goods that are no longer fresh and 
therefore prefers to buy taut and wrinkle-free sausages. 
Attempts have been made to overcome this problem by coating a hydrophilic 
film material, such as cellulose, with a polymer which is impermeable to 
water vapour. In such a multi-layer film, the outside is formed by the 
cellulose, usually a tube of regenerated cellulose material reinforced by 
celloluse fibres, which contains on the inside facing the sausage 
material, a layer of a polymer which is impermeable to water vapour. The 
coating of the cellulose tube may be effected either from the outside or 
from the inside. In the case of coating from the outside the cellulose 
tube must be inverted, as a result of which continuous manufacture of the 
coated tube is not possible. Coating from the inside requires a 
comparatively complicated processing method. 
Another previous proposal consists of using multi-layer composite films 
made of various thermoplastic materials as sausage casings for encasing 
boiling and scalding sausages, but these previously proposed multi-layer 
films of thermoplastic materials, like the single layer films, have the 
undesired property that after the content has cooled they become wrinkled 
and longitudinal folds form because the casing does not exhibit the same 
volume contraction as the filling material on cooling. In the case of 
single or multi-layer films of synthetic materials, such as thermoplasts, 
attempts have been made to overcome this problem by giving the tubular 
films a definite shrinking behaviour by means of a specific stretching 
process following manufacture. It is only possible to obtain a sausage 
free from wrinkles, however, if after cooling, that is at least a few 
hours after the manufacturing process, or better, the next day, it is 
immersed for a few seconds in boiling water. In this reheating to a 
temperature higher than in the scalding treatment, the tubular film 
shrinks again to a certain extent so that even after cooling it surrounds 
the filling material smoothly, without any wrinkles. Such an additional 
hot water treatment, however, called "compressing" by those skilled in the 
art, complicates the manufacture of the sausage and is therefore carried 
out only with reluctance by many manufacturers. 
To obtain a wrinkle-free sausage casing combinations of films have also 
already been proposed, in which a water-impermeable inner layer consists 
of polyamide 11 or 12 and the outer layer of polyamide 6. The 
water-absorbing capacity of the outer layer of such film combinations of 
co-extrudable thermoplasts can be increased by treating the outer layer 
with acid. In such sausage casings, the outer layer shrinks on drying and 
a relatively taut covering of the sausage material is achieved as a 
result. No "compressing" treatment is necessary. The freedom from wrinkles 
of a sausage casing of this type is not, however, optimal. After removing 
the sausage from cold-storage it is necessary to store it at room 
temperature for a relatively long period in order to obtain a reasonably 
wrinkle-free appearance. Accordingly, sausages that have been removed from 
cold-storage and placed straight on the sales counter require some time 
before their appearance is satisfactory. 
Similar considerations apply to sausage casings, also previously proposed, 
that are based on multi-layer films in which the inner layer is a 
physiologically tolerable thermoplast and in which the wrinkle-free fit is 
intended to be achieved by the stiff rubber-elastic properties of the 
outer layer. In addition, both of the above-described sausage casings 
essentially consist of multi-layer films, which are more expensive to 
produce than single films. 
Attempts have also already been made to overcome the problem of achieving a 
wrinkle-free fit of the sausage casing over the sausage material by means 
of a one-layer casing made of polyvinylidene chloride that has been 
stretched to a high degree or made of a copolymer consisting of 70-90% 
vinylidene chloride and 30-10% of vinyl chloride. Such sausage casings 
are, however, also less than optimal as regards freedom from wrinkles. 
Furthermore, polyvinylidene chloride can be extruded only with great 
difficulty. It has only a very narrow melting range of between 140.degree. 
and 145.degree. and splits off CH1, thereby necessitating the use of 
acid-resistant and therefore very expensive extruders. In addition, 
polyvinylidene chloride is thermally unstable. The thermal decomposition 
is auto-catalysed by the hydrochloric acid that is split off. Consequently 
polyvinylidene chloride cannot be extruded as often as desired. In 
addition, because of the stickiness of the film, sausage casing of 
polyvinylidene chloride cannot be produced or stored without using a 
separator material. Also, polyvinylidene chloride is comparatively 
expensive and does not extrude particularly well with other plastics 
materials. 
It is accordingly an object of the present invention to provide a food 
casing, generally a sausage casing, more especially for boiling and 
scalding sausages, which is improved in comparison with previously 
proposed casings and does not have the disadvantages thereof. More 
specifically, it is an object of the invention to provide a casing which 
lies substantially wrinkle-free against the sausage material after the 
boiling or scalding treatment and subsequent cooling, can be manufactured 
cheaply and simply, i.e. is readily extrudable, even together with other 
plastics materials, and can easily be stretched for further improving the 
mechanical properties. 
The present invention provides a food casing, more especially a sausage 
casing, having at least one layer made of a special polyurethane 
elastomer, and also provides the polyurethane itself. Advantageously the 
casing consists of a single layer only. The invention also extends, 
however, to multi-layer casings of which at least one layer, preferably 
the outer or outermost layer, comprises the special polyurethane. Thus, in 
order to improve certain properties of the casing, such as impermeability 
to gas and water, a multi-layer casing can be formed by co-extrusion of 
the special polyurethane elastomer with one or more other materials. 
The polyurethane elastomers used for food casings according to the 
invention, especially sausage casings, are new and combine, in a unique 
manner, the properties of an elastomer (high elasticity) with those of a 
thermoplast (high strength) It is well known that polyurethane elastomers 
will be manufactured mainly from diisocyanates, higher molecular polyols 
(preferably polyester and/or polyether polyols) and lower molecular 
polyols (preferably glycols) as chain-extenders. 
The elastic polyurethane moulding compositions according to the invention 
are characterized in that the polyol and diisocyanate base units consist 
to a substantial extent of linear, aliphatic C.sub.6 -units and the higher 
molecular polyols and diisocyanates are present in an equivalent ratio of 
from approximately 1:3.5 to 1:70, preferably 1:10 to 1:50 and especially 
1:15 to 1:30 and a weight-quantity ratio of approximately 1:0.4 to 1:4.0 
preferably 1:0.6 to 1:3.5 especially 1:0.8 to 1:3.0, together with lower 
molecular weight chain extenders that contain active hydrogen atoms (many 
examples of which are known per se, for example lower molecular glycols), 
in a ratio of equivalents of about 5:1 to 70:1 preferably 7:1 to 60:1 
especially 9:1 to 50:1 based on the higher molecular polyol constituents, 
in which the lower molecular weight chain lengtheners may contain only up 
to approximately 50 mole-% of linear alliphatic C.sub.6 -units. 
Advantageously, the elastic polyurethane moudling compositions used for 
food casings according to the invention have a molecular weight of 2000 or 
more. In the manufacture of the moulding compositions according to the 
invention preferably high molecular polyester and/or polyether polyols 
with a molecular weight of 500 or more are employed. Advantageously, the 
C.sub.6 -units in the higher molecular polyol component of the 
polyurethane are in uniform sequence, that is to say, other fundamental 
units, for example C.sub.2 or C.sub.4 -units should not be present in the 
polyol component to any noticeable extent, whether in regular or irregular 
sequence. It can be advantageous if the chain extenders also contain 
linear, aliphatic C.sub.6 -units in the described manner, wherein the 
proportion of C.sub.6 -units in this case should be up to approximately 50 
mole % only and the rest should be glycols, preferably ethylene glycol, 
1,2 and/or 1,3-propanediol and/or 1,4-butanediol and/or neopentyl glycol. 
Preferably, the polyol component in polyurethane moulding compositions 
contains a polyester, more especially one obtained from 
.epsilon.-caprolactone. Also preferred are moulding compositions in which 
the polyester is obtained from adipic acid and 1,6-hexanediol. 
Hexamethylene diisocyanate is especially suitable as diisocyanate base 
unit in the moulding compositions used in accordance with the invention. 
The polyurethanes of the invention advantageously have base units, more 
especially the polyol and diisocyanate base units, comprising a major 
proportion of linear C.sub.6 aliphatic units, preferably consisting mainly 
of such units. 
The manufacture of polyurethane moulding compositions usable for making 
food casings according to the invention is advantageously carried out in a 
"One-shot process". The constituents chain-extender (e.g., glycols), 
polyol (e.g., hydroxyterminated polyester) and diisocyanate are weighted 
together and heated in the reaction vessel while stirring. After 
completion of the exothermic reaction and cooling, the product, preferably 
cast to form a plate, can be granulated. 
Polyurethanes have in principle not been taken into consideration hitherto 
for sausage casings that are in direct contact with the consumable sausage 
content, since they are generally not resistant to hydrolysis and release 
to the foodstuffs a high proportion of constituents which are forbidden 
according to laws relating to foodstuffs. In addition, elastic 
polyurethanes have hitherto been regarded as unsuitable for sausage 
castings on account of having too pronounced a stickiness during 
processing, since tubular films produced from previous polyurethanes, even 
a relatively long time after extrusion, are so sticky that the flattened 
tubular film sticks together and futhermore, the individual turns of the 
tubular sheet likewise stick together on the supply roll. Consequently the 
handling thereof as sausage casings, particularly the subsequent filling, 
is made extremely difficult. 
Surprisingly, the polyurethane moulding compositions according to the 
invention have a high resistance to hydrolysis. In addition, they possess 
a high resistance to other solvents, and to fats, and release to the 
surrounding medium only infinitely small amounts of their constituents, 
thus meeting the legal requirements regarding foodstuffs. Furthermore, the 
polyurethane moulding compositions used according to the invention can be 
extruded to form food casings without any undesirable sticking. In spite 
of the low tendency to stickm the moulding compositions of the invention 
can nevertheless be co-extruded very well with other thermoplasts to form 
multi-layer films (owing to their broad melting range of approximately 
180.degree. to 220.degree. C.,) if a modification of the properties of the 
single sheet film is desired. An important factor is that coextrusion is 
possible without using an adhesion promoter. As a result of the relatively 
broad melting range, the polyurethane moulding compositions according to 
the invention are particularly suitable for the blow extrusion preferred 
for the manufacture of sausage casings. Owing to their sepcial properties, 
the moulding compositions according to the invention meet the following 
two essential requirements for processing in the blow extrusion process: 
1. faultless drawing and inflation to form a tubular film on a continuous 
process and 
2. the sheet can be turned round and wound up, without sticking, only a few 
seconds after emerging from the nozzle. 
A sausage casing according to the invention fits practically wrinkle-free 
after scalding/boiling and re-cooling, and there is virtually no 
detachment of the sausage material from the wall of the sausage casing. 
These proerties stem, surprisingly, from the unique combination of elastic 
properties and strength of the polyurethane moulding compositions used as 
starting material for the sausage casing according to the invention. Some 
of the most important properties of the polyurethane moulding 
compositions, as regards use as a sausage casing material, lie in the 
following range: 
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Shore hardness 
50 to 80, preferably 60 to 70 
Tensile strength 
measured in a film 50.mu. thick 
(N/mm.sup.2) 25 to 80, preferably 30 to 70 
Elongation at break (%) 
100 to 400, preferably 150 to 350 
Stress value at 50% 
elongation (N/mm.sup.2) 
15 to 50, preferably 20 to 40 
Stress value at 100% 
elongation (N/mm.sup.2) 
20 to 45, preferably 25 to 35 
shear modulus at 0.degree. C 
4000 - 5000 kp/cm.sup.2 
preferably 4400 - 4600 kp/cm.sup.2 
in particular approximately 4500 kp/cm.sup.2 
shear modulus at 20.degree. C 
2000 - 3000 kp/cm.sup.2 
preferably 2500 - 2700 kp/cm.sup.2 
in particular approximately 2600 kp/cm.sup.2 
Glass temperature 
-30 to -70, preferably -40 to -60.degree. C 
______________________________________ 
As a result of the high elasticity, which the polyurethanes of the 
invention exhibit even at temperatures below room temperature (low 
transition temperature) a wrinkle-free fit of the sausage casing is 
guaranteed also at relatively low temperatures, even as far as below room 
temperature. The good strength properties enable a further requirement 
made of the sausage casing to be met, namely its dimensional stability, as 
a result of which pear-shaped buckling is avoided. 
The manufacture of a food (especially sausage) casing according to the 
invention is effected by extrusion of the polyurethane moulding 
compositions, preferably according to the blow extrusion process. The 
moulding compositions can be heated and shaped on a known blow extruder. 
The properties of the extruded casing may be further improved by biaxial 
stretching. The longitudinal stretching is advantageously carried out with 
stretching ratios in the range of approximately 1:1 to 1:4, preferably 
from 1:1.5 to 1:3, whilst the transverse stretching is also preferable 
carried out with stretching ratios in the range of approximately 1:1 to 
1:4. 
As already mentioned, the polyurethane moulding compositions have a 
comparatively broad melting range. This can be controlled as desired in 
the range of from approximately 150.degree. to 250.degree. C., for example 
by varying the ratio of the NCO-groups to the OH-groups in the starting 
components of the polyurethane within a small range, for example between 
1.00 and 0.95. Owing to this broad melting range, polyurethane moulding 
compositions of the invention may be coextruded with other plastics 
materials, for example with polyethylene, PVC, PVDC, polyamides and 
polyesters, without adhesion promoters, to form two-layer or three-layer 
films, provided of course that the processing range of the other 
thermoplasts lies in the processing range of the polyurethane moulding 
compositions used in accordance with the invention. By the combination 
with other plastics materials, it is possible to increase even further the 
advantages of the polyurethanes used according to the invention. 
The thickness of casings according to the invention may be in the range of 
from 20.mu. to 150.mu.. Typical plastics material sausage casings have a 
thickness of approximately 30 to 70 .mu.m, preferably 40 to 50 .mu.m. In 
the case of multilayer films the ratio of the thickness of the inner layer 
to that of the outer layer may be in the range of from approximately 1:1 
to 1:6, and a ratio of 1 (inner layer) to 3 (outer layer) should prove 
optimal for most types of sausage. The thickness of multi-layer films is 
typically about 40 to 60 .mu.m and is therefore of the same order as the 
customary one layer films. 
To summarise, casings according to the invention offer the following 
advantages: 
1. Owing to their adequate crystallisability they can be wound up 
immediately after extrusion and to not stick or clog. 
2. Equally beneficial they have a very high elasticity, which can be 
further improved by biaxial stretching. 
3. Their mechanical properties are, throughout, generally better than the 
conventional plastics such as, for example, sheets of polyethylene, 
polyamide, polyvinylchloride, and unstretched polyester. A further 
considerable increase in strength can be achieved by biaxial stretching. 
4. The starting material for the polyurethane sausage casings according to 
the invention is currently cheaper than that for the previously proposed 
casings of polyamide 12 or polyvinylidene chloride. 
5. The chemistry of the starting material renders possible a broad 
modification of the tubular casing as desired, for example softer and 
harder types, higher and lower elasticity, easy colourability, processing 
without further additives, etc. 
6. Since the polyurethane casing can be produced without using additives, 
the risk of monomeric additive substances that are considered legally 
harmful exuding into the food material (e.g. sausage meat) is practically 
non-existent. 
7. The casing can be made with a minimal permeability to gas and a low 
permeability to water vapour. 
8. The casing according to the invention is, very resistant to hydrolysis 
especially in comparison with known polyurethane films, and therefore 
suitable as a casing for boiling and scalding sausage. 
9. The casing according to the invention lies taut and absolutely 
wrinkle-free against the food material content. 
10. It has a supple, soft feel. 
11. The casings according to the invention have little or no smell or 
taste, and can be easily extruded over a wide temperature range (say, 
180.degree. to 235.degree. C.). It is precisely in this property that 
casings according to the invention are superior to the known 
polyvinylidene chloride sausage casings which not only have an unpleasant 
smell, since they contain additives and anti-clogging agents, but also can 
be extruded only with difficulty and within narrow temperature limits. 
12. The casings receive print well. 
13. The casings may be heat-sealed and fused. In comparison with other 
plastics films they have good adhesion and can therefore be coextruded 
very well. 
14. The casings according to the invention can be harmless to the 
environment since when waste is destroyed by combustion offensive chlorine 
compounds are in general not evolved. By contrast, the previously proposed 
polyvinylidene chloride casings split off hydrochloric acid when heated. 
15. The casings have such a good tear propagation resistance that steeping 
is possible. 
Although the following Examples refer primarily to the polyurethane of the 
invention and their use in sausage casings, similar considerations will 
apply to other food casings.

The following Examples illustrate the invention: 
(A) MANUFACTURE OF THE POLYURETHANE MOULDING COMPOSITION USED AS STARTING 
MATERIAL 
Example I 
Composition of the PUR-starting mixture (PUR--elastic polyurethane moulding 
composition used in accordance with the invention) 
24.00 parts by weight of polycaprolactone having a molecular weight of 2000 
(H.sub.2 O content: 0.03%) 
50.40 parts by weight of 1,6-hexamethylene-diisocyanate 
26.46 parts by weight of 1,4-butanediol 
Equivalent ratio of higher molecular polyol to diisocyanate = 1:25 
Equivalent ratio of chain-extender to higher molecular polyol = 24.5:1 
Method of manufacture 
The manufacture is effected in a one-shot process. 1,4-Butanediol, 
polycaprolactone and diisocyanate are heated to 60.degree. C., while 
stirring, in a reaction vessel. As a result of exothermic reaction the 
temperature increases to 240.degree. C. in approximately 10 minutes. At 
that temperature, the product is cast onto a polytetrafluoroethylene film. 
After approximately 5 hours the plate can be granulated. 
Example II 
Composition of the PUR-mixture 
27.00 parts by weight of polycaproplactone having a molecular weight of 
2000 (H.sub.2 O content: 0.03%) 
50.40 parts by weight of 1,6-hexamethylene-diisocyanate 
26.32 parts by weight of 1,4-butanediol 
Equivalent ratio of higher molecular polyol to diisocyanate = 1.22:2 
Equivalent ratio of chain-extender to higher molecular polyol = 21.7:1 
The manufacture is carried out as described in Example I. 
Example III 
Composition of the PUR-mixture 
24.00 parts by weight of polycaprolactone having a molecular weight of 4000 
(H.sub.2 O content: 0.03%) 
50.40 parts by weight of 1,6-hexamethylene-diisocyanate 
27.00 parts of weight of 1,4-butanediol 
Equivalent ratio of higher molecular polyol to diisocyanate = 1:50 
Equivalent ratio of chain-extender to higher molecular polyol = 50:1 
The manufacture is carried out as described in Example I. 
Example IV 
Composition of the PUR-mixture 
30.00 parts by weight of polycaprolactone having a molecular weight of 1000 
(H.sub.2 O content: 0.03%) 
50.40 parts by weight of 1,6-hexamethylene-diisocyanate 
24.84 parts by weight of 1,4-butanediol 
Equivalent ratio of higher molecular polyol to diisocyanate = 1:10 
Equivalent ratio of chain-extender to higher molecular polyol = 9.2:1 
The manufacture is carried out as described in Example I. 
(B) MANUFACTURE OF SAUSAGE CASINGS ACCORDING TO THE INVENTION 
The moulding compositions of the Examples (A) I to IV are extruded 
according to the blow-film process, in a manner known per se, to form 
one-layer, unstretched sausage casings, the moulding compositions I and 
III as well as II and IV being extruded, respectively, under the 
conditions described below: 
Moulding compositions I + III 
The predried polyurethane granulate having a residual moisture content of 
0.05% is extruded on a Reifenhauser extruder having a screw diameter of 30 
mm, a screw length of 20 D, a retro extruder head and a nozzle opening of 
1.0 mm. The extruder has no parts that are especially protected against 
corrosion. Looking in the direction of extrusion the following 
temperatures are established: 200.degree. C., 210.degree. C., 220.degree. 
C., 225.degree. C., 220.degree. C. The screw conveys at a speed of 60 
rev/min. The output of melt obtained thereby permits a discharge speed of 
12 m/min, the mean wall thickness of the extruded casing being 
approximately 50 .mu.m. The diameter of the tube is 60 mm. The 
fluctuations in the wall thickness are limited by adjusting the nozzle 
opening to .+-.8%. The melt is cooled to below the solidification point 
with cooled air, drawn-off, flattened and wound up. Drawing off and 
winding up are carried out in the conventional manner. 
The length of the air bubble between the extruder nozzle and the nip roller 
is approximately 2 m. The flattened tube can be wound up immediately. The 
risk of clogging, even without injecting a separator, for example an oil 
or dust suitable for foodstuffs, does not arise. 
Moulding compositions II + IV 
The predried polyurethane granulate having a residual moisture of 0.05% is 
extruded on a Reifenhauser extruder having a screw diameter of 20 mm, a 
screw length of 20 D, a retro extruder head and a nozzle opening of 1.0 
mm. The extruder has no parts that are especially protected against 
corrosion. Looking in the direction of extrusion, the following 
temperatures are established: 180.degree. C., 190.degree. C., 200.degree. 
C., 215.degree. C., 210.degree. C. The screw conveys at a speed of 50 
rev/min. The output obtained thereby permits a draw-off speed of 10 m/min 
the mean wall thickness being 45 .mu.m .+-. 8% and the tube diameter 
amounting to 60 mm. The melt is cooled to below the solidification point 
with cooled air. The air bubble as far as the nip roller has a length of 2 
m. The flattened tube may be wound up immediately, without there being any 
risk of clogging. 
Relevant physical properties of the films I' to IV' obtained from the 
moulding compositions I to IV are set out in Table I below: 
TABLE I 
__________________________________________________________________________ 
Sausage casing I' 
Sausage casing II' 
Sausage casing III' 
Sausage casing IV' 
longi- longi- longi- longi- 
tudinal 
transverse 
tudinal 
transverse 
tudinal 
transverse 
tudinal 
transverse 
__________________________________________________________________________ 
Tensile strength (N/mm.sup.2) 
50 35 42 36 36 36 48 37 
Stress value at 50 % 
Elongation (N/mm.sup.2) 
30 23 27 24 26 26 28 24 
Stress value at 100 % 
Elongation (N/mm.sup.2) 
35 26 32 26 29 27 34 25 
Elongation at break (%) 
260 250 200 350 180 300 200 330 
__________________________________________________________________________ 
Surprisingly, sausage casings produced from the polyurethane moulding 
compositions used in accordance with the invention show unexpectedly high 
resistance to the action of water, dilute acetic acid, 10% ethanol and 
coconut oil. Table II below shows the results of migration tests on a 
sausage casing according to Example I. The individual migration tests were 
carried out under the conditions laid down by the Bundesgesundheitsamt 
(BGA) (Federal Health Office) (Bundesgesundheitsblatt, 1970, No. 14, page 
203-204, note 15). The test showed that in each case less than the 
permissible amount, i.e. less than 6 mg of migrating substance, was 
released per dm.sup.2 of film surface (Supplement to Kunsstoffempfehlung 
of 1.31975). Consequently, subject to the express agreement of the BGA, 
the requirements to be met by plastics materials in the foodstuff trade 
are fulfilled. 
TABLE II 
__________________________________________________________________________ 
Migration tests foodstuff simulants* 
test test distilled 
3 % by wt, 
10 % by vol. 
purified 
temperature 
duration 
water 
CH.sub.3 COOH 
C.sub.2 H.sub.5 OH 
coconut oil 
Test material 
(.degree. C) 
(hours) 
** *** **** ***** 
__________________________________________________________________________ 
Sausage casing 
40 10 .times. 24 
2.9 mg/dm.sup.2 
according to 
Example I 
70 2 3.8 mg/dm.sup.2 
Calibre 50 .phi. 
Wall thickness 
100 1/2 5.5 mg/dm.sup.2 
4.2 mg/dm.sup.2 
4.2 mg/dm.sup.2 
43 - 47 .mu. 
121 1/2 4.6 mg/dm.sup.2 
__________________________________________________________________________ 
*:The following extraction agents are used as foodstuff simulants 
**:distilled and de-ionised water (with specific conductivity data and 
blank test) 
***:3% by weight of acetic acid (CH.sub.3 COOH) (starting from a degree o 
purity p.a. per analysis) 
****:10% by volume of ethyl alcohol (C.sub.2 H.sub.5 OH) (starting from a 
degree of purity p.a. per analysis): 
*****:purified.sup.1 coconut oil (1 purification of coconut oil; of. 
Bundesgesundheitsblatt 1970, No. 14 page 203-204) 
Ground nut oil or triglycerides (esters of glycerol with fatty acids of 
medium chain length) 
The calibration of the specimen is to be determined by a blank test. 
A further surprising advantage of sausage casings according to the 
invention in comparison with the conventional sausage casings of 
thermoplastic synthetic materials, for example, polyamide, resides in the 
fact that the strength properties at elevated temperatures that correspond 
approximately to the temperatures of the boiling or scalding treatment, 
are reduced by a much lesser extent than the corresponding values of the 
polyamide. In table III below, .delta..sub.s at 20.degree. C. and 
80.degree. C. for sausage casings of polyurethane according to the 
invention, and for sausage casings of polyamide 12, are compared. 
TABLE III 
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PUR polyamide 12 
unstretched unstretched 
20.degree. C 
longitudinal 
transverse 
longitudinal 
transverse 
______________________________________ 
.delta..sub.s (N/mm.sup.2) 
28 30 36 30 
.epsilon..sub.s (%) 
22 10 9 8.5 
PUR polysamide 12 
unstretched unstretched 
80.degree. C 
longitudinal 
transverse 
longitudinal 
transverse 
______________________________________ 
.delta..sub.s (N/mm.sup.2) 
10 15 8 7.8 
.epsilon..sub.s (%) 
16 14 17 17 
______________________________________ 
The significance of these results is that sausage casings according to th 
invention do not deform on scalding. C) 
Manufacture of a biaxially stretched, one-layer sausage casing: 
The biaxial stretching is carried out by methods known per se. 
The significance of these results is that sausage casings according to the 
invention do not deform on scalding. 
(C) MANUFACTURE OF A BIAXIALLY STRETCHED, ONE-LAYER SAUSAGE CASING 
The biaxial stretching is carried out by methods known per se. 
Predried granulated polyurethane made as described in Example I and having 
a residual moisture content of 0.05% is extruded on a Reifenhauser 
extruder having a screw diameter of 30 mm, a screw length of 20 D, a retro 
extruder head and a nozzle opening of 1.0 mm. The extruder has no parts 
that are especially protected against corrosion. Looking in the direction 
of extrusion, the following temperatures are established: 200.degree. C., 
210.degree. C., 220.degree. C., 225.degree. C. and 220.degree. C. The 
screw conveys at a speed of 60 rev/min. The emerging melt is cooled. By 
means of an air line passing through the extruder head, the cooled tube is 
inflated. The further transport of the tubular film, having a thickness of 
approximately 200 .mu.m, is effected by a pair of rollers after which 
there is arranged a heating path for reheating. There the casing is 
brought continuously to the stretching temperature of approximately 
105.degree. C. As a result of the compressed air supplied, the casing is 
inflated radially in the ratio 1:2, whilst being simultaneously stretched 
in the longitudinal direction in the ratio of 1:2 by a second pair of 
rollers arranged after the heating device in the direction of transport, 
which pair of rollers draws off the casing at a speed which is double that 
of the first pair of roller and simultaneously nips it. The transverse 
stretching is controlled in accordance with the diameter by means of the 
enclosed volume of air, using a pressure regulating valve. The final wall 
thickness amounts to 45 .mu.m .+-. 10%. The diameter of the stretched 
casing is 60 mm. The film thus obtained is subjected to a traction test 
according to DIN 53455 at 20.degree. and 80.degree. C. The values obtained 
for the stretching stress and elongation are compared in Table IV below 
with the corresponding values for an unstretched casing according to the 
invention. The results of the traction tests show that a surprisingly 
large improvement in tensile strength, i.e. 3 to 4 times that of the 
casing in the unstretched state, is obtained by means of the described 
stretching. Furthermore, the proportion of amorphous regions is reduced by 
the stretching, so that impermeability to gas, as well as to water and 
fat, is increased considerably. Since the molecular orientation is frozen 
in the film by means of intense cooling directly after stretching an 
improved transparency is obtained. During the manufacture at 70.degree. to 
85.degree. C. of scalding and boiling sausages, shrinking forces are 
released which makes it possible during cooling for the sausage casing to 
follow the volume contraction that takes place in the sausage material 
until the cold storage temperature is reached. 
TABLE IV 
__________________________________________________________________________ 
1. at 20.degree. C 
PUR according to 
PUR according to 
Example 1 Example I 
unstretched stretched PVd C stretched 
longi- longi- longi- 
tudinal 
transverse 
tudinal 
transverse 
tudinal 
transverse 
__________________________________________________________________________ 
Yield stress 
28 30 95 65 55 93 
.delta. [N/mm.sup.2 ] 
elongation 
.epsilon. (%) 
22 10 62 45 68 50 
2. at 80.degree. C 
PUR according to 
PUR according to 
Example 1 Example I 
unstretched stretched PVdC stretched 
longi- longi- longi- 
tudinal 
transverse 
tudinal 
transverse 
tudinal 
transverse 
Yield stress 
10 15 51 48 40 44 
.delta. [N/mm.sup.2 ] 
elongation 
16 14 70 60 88 45 
.epsilon. [%] 
__________________________________________________________________________ 
(D) MANUFACTURE OF MULTI-LAYER SAUSAGE CASINGS BY COEXTRUSION WITH OTHER 
PLASTICS MATERIALS 
Polyurethane can be coextruded inter alia with polyacrylonitrile 
copolymers, polyamide and polyethylene. 
In order to improve the impermeability to water-vapour and oxygen in an 
unstretched polyurethane casing, the above materials are combined with PUR 
according to the invention. 
EXAMPLE 1 PUR/polyamide 12 
PUR according to Example I on the outside and PA (polyamide) 12 on the 
inside are coextruded on a Baroflex plant according to U.S. Pat. No. 
3,642,397. 
______________________________________ 
Extr. I PUR according to Example I 
screw diameter 30 mm 
screwlength 21 D 
rotational speed 60 min.sup.-1 
cylinder temperature 220.degree. C 220.degree. C 
adapter 240.degree. C 
Extr. II PA 
screw diameter 30 mm 
screw length 21 D 
rotational speed 40 min.sup.-1 
cylinder temperature 280.degree. C 270.degree. C 
adapter 250.degree. C 
head 220.degree. C 
nozzle 225.degree. C 
______________________________________ 
The two melts are combined outside the nozzle. The tube is cooled down by 
blowing air against the film and conveyed by way of a pair of draw-off 
rollers to the winding up station. The wall thickness of the film is 
50.mu., PUR being 35.mu. and PA 12 15.mu. thick. 
______________________________________ 
Gas permeability of the 50 .mu. thick composite tubular film 
Measured 
Test with Measuring process 
value Unit 
______________________________________ 
Water vapour 
DIN 53122 4.5 g/day.m.sup.2 
(20.degree. C/85% RH) 
Water vapour 
BS 1133 20 g/day.m.sup.2 
(30.degree. C/90% RH) 
Nitrogen ASTM D 1434 0.8 cm.sup.3 bar/ 
day.dm.sup.2 
oxygen ASTM D 1434 3.5 cm.sup.3 bar/ 
day.dm.sup.2 
Carbon dioxide 
ASTM D 1434 10 cm.sup.3 bar/ 
day.dm.sup.2 
______________________________________ 
The strength of adhesion of the two film layers is judged moderate to good 
and is adequate for the sausage casing according to the invention. 
EXAMPLE 2 PUR/Barex 
PUR outside, polyacrylonitrile copolymer with methyl acrylate and butadiene 
(trade name "Barex", Lonza AG) inside. 
______________________________________ 
Extr. I PUR according to Example I 
screw diameter 30 mm 
screw length 21 D 
rotational speed 60 min.sup.- 1 
cylinder temperature 210.degree. C 215.degree. C 
adapter 215.degree. C 
Extr. II Barex 
screw diameter 30 mm 
screw length 21 D 
rotational speed 30 min.sup.-1 
cylinder temperature 180.degree. C 195.degree. C 
adapter 200.degree. C 
head 200.degree. C 
nozzle 205.degree. C 
______________________________________ 
The two melts are combined outside the nozzle. The tube is cooled down by 
blowing air against the film and conveyed by way of a pair of draw-off 
rollers to the winding up station. The wall thickness of the film is 
50.mu., PUR being 35.mu. and Barex 15.mu. thick. 
______________________________________ 
Gas permeabiity of the 50 .mu. thick composite film 
Meas- 
Measuring ured 
Tested with 
process value Unit 
______________________________________ 
Water vapour g. 0.001 in/ 
(38.degree. C/90% RH) 
ASTM E 96-63/ 
5.8 100 in.sup.2.24hr 
method E 
Oxygen ASTM D 1434 1.0 cm.sup. 3 .0.001 in/ 
(23.degree. C) 100 in.sup.2.24hr.atm 
Carbon dioxide 
ASTM D 1434 1.5 cm.sup. 3 .0.001 in/ 
(23.degree. C) 100 in.sup. 2.24hr.atm 
Nitrogen ASTM D 1434 0.4 cm.sup. 3.0.001 in/ 
(23.degree. C) 100 in.sup. 2 .24hr.atm 
______________________________________ 
The adhesion between the two layers is better than in Example 3 but not as 
good as in Example 1. 
EXAMPLE 3 PUR/Polyethylene 
PUR outside, polyethylene inside. 
______________________________________ 
Extr. I PUR according to Example I 
screw diameter 30 mm 
screw length 21 D 
rotational speed 60 min.sup.-1 
cylinder temperature 
210.degree. C, 215.degree. C 
adapter 215.degree. C 
Extr. II Polyethylene 
Density: 0.94 - 0.96 
screw diameter 30 mm 
screw length 21 D 
rotational speed 30 min.sup.-1 
cylinder temperature 
180.degree. C, 200.degree. C 
adapter 200.degree. C 
head 205.degree. C 
nozzle 210.degree. C 
______________________________________ 
The two melts are combined outside the nozzle. The tube is cooled down by 
blowing air against the film and conveyed by way of a pair of draw-off 
rollers to the winding up station. The wall thickness of the film is 
50.mu., PUR being 35.mu. and polyethylene 15.mu. thick. 
______________________________________ 
Gas permeability of the 50 .mu. thick composite film 
Meas- 
Measuring ured 
Tested with 
process value Unit 
______________________________________ 
Water vapour 
ASTM E 96-63 0.3 g. 0.001 in/ 
(38.degree. C/90% RH) 
method E 100 in.sup.2.24hr 
Oxygen ASTM D 1434 180 g. 0.001 in/ 
(23.degree. C) 100 in.sup.2.24hr.atm 
Carbon dioxide 
ASTM D 1434 500 g. 0.001 in/ inu -(23.degree. C) 100 
in.sup.2.24hr.atm 
not 
Nitrogen ASTM D 1434 measured not measured 
(23.degree. C) 
______________________________________ 
The strength of adhesion of the two layers is rather low, but is adequate 
for a sausage casing.