Packaging cloth

A packing cloth comprising a woven or knitted fabric obtained by using, as at least a portion of either or both of warp and weft, thread or tape obtained by melt extrusion of a linear low-density ethylene polymer having a density of less than 0.945 g/cm.sup.3 and having a branched short chain, followed by stretching at a temperature of less than 120.degree. C. This packaging cloth has an improved tear strength and notch propagation resistance.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a low temperature shrinkable packaging 
cloth suitable for use in shrink packaging of articles to be packaged. 
2. Description of the Prior Art 
Heretofore, heat-shrinkable films derived from, for example, polyvinyl 
chloride and polyethylene having a branched long chain have been generally 
used in shrink packaging or wrapping. These heat-shrinkable films are, 
however, poor in mechanical strength such as tear strength, tensile 
strength, and hardwearing properties and have notch propagation 
properties. Therefore, there are ongoing problems during storage or 
transportation wherein the packaging films are broken at the corner or 
edge portions of articles that are packaged. For these reasons, the 
above-mentioned known heat-shrinkable films have not been substantially 
used in heavy duty packaging but are used in light duty packaging in which 
the packaging film is subjected to a relatively small mechanical stress. 
Other problems of the above-mentioned heat shrinkable packaging films are 
that the shrinkable temperature is high and, therefore, the contents to be 
packaged are limited, and the direction of shrinkage is limited. For 
example, free selection of the direction and the amount of shrinkage such 
as (i) M-direction (i.e., "machine-direction") ultra-high shrinkage and 
T-direction (i.e., "transverse direction") low shrinkage, (ii) M-direction 
ultra-high shrinkage and T-direction ultra-high shrinkage, and (iii) 
M-direction low shrinkage and T-direction ultra-high shrinkage depending 
upon the shapes and conditions of articles to be packed cannot be 
attained. Further, it is impossible to expect that the shrinkage behavior 
in only a portion of the entire width in M-direction or T-direction is 
changed. Accordingly, the above-mentioned known heat shrinkable films can 
shrink-package articles having simple shapes, but cannot desirably 
shrink-package articles having complicated or not simple shapes. 
Furthermore, the maximum width of the above-mentioned heat shrinkable films 
is limited to 1000 mm to 1500 mm under present film forming techniques. 
For this reason, the above-mentioned shrinkable films have not been used 
in heavy duty packaging. 
SUMMARY OF THE INVENTION 
Accordingly, the objects of the present invention are to eliminate the 
above-mentioned problems of the prior art and to provide a packaging cloth 
having a improved mechanical strength such as tear strength, improved 
notch propagation properties, and shrinkable at a relatively low 
temperature. 
Other objects and advantages of the present invention will be apparent from 
the following description. 
In accordance with the present invention, there is provided a packaging 
cloth comprising a woven or knitted fabric obtained by using, as at least 
a portion of either or both of warp and weft for a woven fabric or at 
least a portion of either or both of wales and courses for a knitted 
fabric, thread or tape obtained by melt extrusion of linear low-density 
ethylene polymer having a density of less than 0.945 g/cm.sup.3 and having 
a branched short chain, followed by stretching at a temperature of less 
than 120.degree. C. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Shrinkable Thermoplastic Resin 
The heat shrinkable thermoplastic resins usable in the present invention 
include, for example, linear low-density ethylene polymers having a 
branched short chain and mixtures thereof with high-density ethylene 
polymers or propylene polymers. 
The linear low-density ethylene polymers having a branched short chain must 
have a density of less than 0.945 g/cm.sup.3, preferably less than 0.935 
g/cm.sup.3. These ethylene polymers may be copolymers of a major amount (e 
g., more than 50% by weight) of ethylene with a minor amount (e.g., less 
than 50% by weight) of an alpha-olefin such as propylene, butene-1, 
hexene-1, 4-methylpentene-1, and octene-1. These polymers can be produced 
in any conventional polymerization manner, e.g., vapor phase 
polymerization methods, slurry polymerization methods, or solution 
polymerization methods, in the presence of a transition-metal compound and 
an organic-metal compound catalyst. 
The density of the linear low-density ethylene polymer must be less than 
0.945 g/cm.sup.3, preferably less than 0.935 g/cm.sup.3 but not less than 
0.905 g/cm.sup.3. A density of the ethylene polymers of 0.945 g/cm.sup.3 
or more results in a decrease in the heat shrinkability, which in turn 
decreases shrink force and causes very poor shrink-packaging ability, 
although the strength of the cloth is not adversely affected. Contrary to 
this, a density of the ethylene polymers of less than 0.945 g/cm.sup.3 can 
solve the heat shrinkability and the other problems. This is greatly 
improved by the use of the linear low-density ethylene polymers having a 
density of less than 0.935 g/cm.sup.3. However, the use of the linear 
low-density ethylene polymers having a density of less than 0.905 
g/cm.sup.3 tends to cause a problem in the stretchability of the film. 
The linear low-density ethylene polymers mentioned above must have a 
branched short chain. Although the length of the short chain is not 
specifically limited, the branched chains having 10 or less carbon atoms 
are preferable in the present invention. The use of, for example, 
low-density ethylene polymers having a branched long chain is not 
preferable because the stretchability and the heat shrink rate tend to 
deteriorate. 
Although no special limitation is present in the melt flow rate (MFR) at 
190.degree. C. under 2.16 kg of the linear low-density ethylene polymer, 
the preferable MFR of the linear low-density ethylene polymers is 2.0 g/10 
min or less in view of the strength and the heat shrinkability and is 0.1 
g/10 min or more in view of the forming and the stretchability. Also, 
there is no limitation in a ratio of a high load melt flow rate (HLMFR at 
190.degree. C. under 21.6 kg) to a melt flow rate (MFR at 190.degree. C. 
under 2.16 kg) (i.e., HLMFR/MFR) of the linear low-density ethylene 
polymer, which ratio relates to the stretchability, and the strength is 
preferably 40 or less. 
According to the present invention, the mixtures of (A) 25 to 90 parts by 
weight of the above-mentioned linear low-density ethylene polymers having 
a density of less than 0.945 g/cm.sup.3 and having a branched short chain 
and (B) 10 to 75 parts by weight of a high-density ethylene polymer having 
a density of not less than 0.945 g/cm.sup.3 can be used as the 
above-mentioned resin. 
The high-density ethylene polymers (B) used in the present invention having 
a density of not less than 0.945 g/cm.sup.3 may be ethylene homopolymers 
and copoplymers of a major amount (e.g., more than 50% by weight) of 
ethylene and a minor amount (e.g., less than 50% by weight) of an 
alpha-olefin such as propylene, butene-1, hexene-1, 4-methylpentene-1, and 
octene-1. These polymers or copolymers can be produced in any conventional 
polymerization manner, e.g., slurry polymerization methods, or solution 
polymerization methods, in the presence of a transition-metal compound and 
an organic-metal compound catalyst. 
The density of the high-density ethylene polymers (B) must be 0.945 
g/cm.sup.3 or more. When the density of the high-density ethylene polymers 
is less than 0.945 g/cm.sup.3, the stretchability and the strength of the 
mixtures becomes poor. The MFR of the ethylene polymers (B) is preferably 
2 to 0.1 g/10 min and the ratio of HLMFR/MFR of the ethylene polymers (B) 
is desirably less than 40. 
A weight ratio of the linear low-density ethylene polymers (A)/ the 
high-density ethylene polymer (B) is preferably 25-90/10-75, more 
preferably 30-60/40-70. When the amount of the linear low-density ethylene 
polymers (A) in the mixtures is more than the upper limit of the 
above-mentioned range, the stretchability and the strength become poor. 
Contrary to this, when the amount of the linear low-density ethylene 
polymers (A) in the mixtures is less than the lower limit of the 
above-mentioned range, the heat shrinkability decreases and, therefore, 
the shrink force decreases and the shrink-packaging ability becomes very 
poor. Furthermore, in the latter case, the stress relaxation and the creep 
resistance become worse and, therefore, the shrink force are relaxed 
during storage over a period of time. 
Furthermore, the mixtures of the ethylene polymers (A) and (B) preferably 
have (i) a density of 0.920 g/cm.sup.3 or more in view of the 
stretchability and the strength, (ii) a density of less than 0.945 
g/cm.sup.3 in view of the heat shrinkability, the creep resistance, and 
the heat sealing properties, (iii) an MFR of 2 g/10 min or less in view of 
the strength, (iv) a ratio of HLMFR/MFR of 10 or more in view of the 
formability, and (v) a ratio of HLMFR/MFR of 40 or less in view of the 
stretchability and the strength. 
According to the present invention, the mixtures of (A) 20 to 90 parts by 
weight of a linear low-density ethylene polymer having a density of less 
than 0.945 g/cm.sup.3 and having a branched short chain and (C) 10 to 80 
parts by weight of a propylene polymer can also be used as the 
above-mentioned heat shrinkable resin. 
The propylene polymers (C) usable in the present invention may be propylene 
homopolymer and copolymers of a major amount (e.g., more than 50% by 
weight) of propylene and a minor amount (e.g., less than 50% by weight) of 
an alpha-olefin such as ethylene. Preferable propylene polymers (C) are 
propylene homopolymers (i.e., polypropylenes) having an MFR of 0.5 to 8 
g/10 min, more preferably 1 to 5 g/10 min, in view of the improvement in 
the shrink stress retention ability. Although there in no limitation in 
the MFR of the propylene polymers, the MFR of the propylene polymer is 
preferably 0.5 g/10 min or more in view of the formability and the 
stretchability but preferably 8 g/10 min or less in view of the heat 
shrinkability, when the blend amount of the propylene polymers in the 
mixture is relatively large. 
In the present invention, a weight ratio of the linear low-density ethylene 
polymers (A)/the propylene polymers (C) is preferably 20-90/10-80, more 
preferably 40-70/30-60. When the amount of the linear low-density ethylene 
polymers (A) in the mixtures is more than 90 parts by weight, the 
shrinkage stress tends to decrease and to cause a problem in packaging, as 
the temperature of the shrunk cloth decreases from the predetermined 
shrink packaging temperature at an oven to ambient temperature, although 
there is no problem in the shrinkage amount. On the other hand, when the 
amount of the linear low-density ethylene polymers (A) in the mixtures is 
less than 20 parts by weight, the overall shrinkage amount tends to 
decrease and, therefore, the shrink packaging must be carried out at a 
remarkably high temperature, although there is no problem in the retention 
of the shrinkage stress. 
Preparation of Thread or Tape 
According to the present invention, the heat shrinkable thermoplastic 
resins are converted or extruded into threads or tapes. These threads or 
tapes can be prepared in any conventional manner, for example, an 
inflation method. 
In the practice of the extrusion, the thermoplastic resins are extruded 
under a molten state through a slit of a die in the form of films and, 
then, after cooling, the extruded films are stretched at a temperature of 
less than 120.degree. C., preferably 70.degree. C. to 110.degree. C. at a 
high stretching ratio of, for example, 2 to 9 times. Thus, the desired 
threads or tapes are prepared. 
In the preparation of the threads or tapes, when the stretching temperature 
is 120.degree. C. or more, not only does the stretchability become poor, 
but also there is increased slip between the molecule chains, which causes 
ineffective orientation during the stretching operation. Thus, the desired 
strength and heat shrinkage factor cannot be obtained. For this reason, 
the stretching temperature of less than 120.degree. C. is used in the 
present invention. However, the stretching temperature of less than 
70.degree. C. tend to cause turning white, the decrease in the various 
properties, or the decrease in the stretchability of the threads or tapes. 
For this reason, the preferable stretching temperature is 70.degree. C. to 
110.degree. C. Especially preferable is a stretching temperature of 
85.degree. C. to 105.degree. C., at which the best stretchability and the 
most balanced desired properties can be obtained. 
The stretching ratio of the extruded threads or tapes according to the 
present invention is preferably 2 to 8, more preferably 4 to 7, although 
the desired stretch ratio depends upon, for example, the desired strength 
and heat shrinkability. When the stretching ratio is less than 2, problems 
in the strength and heat shrinkability of the resultant threads or tapes 
are likely to occur. Contrary to this, a stretching ratio of more than 8 
tends to cause problems in the stretchability. Furthermore, in order to 
decrease the natural shrinking properties after the stretching and to 
prevent the collapse of paper cores at winding, a thermal treatment 
immediately after the stretching may be, or be preferably, carried out 
under the conditions where the dimensions of the stretched threads or 
tapes are fixed. 
Preparation of Woven or Knitted Fabric 
According to the present invention, the desired shrink-packaging cloth can 
be obtained by using, as at least a portion of warp, at least a portion of 
weft, or at least a portion of both warp and weft for a woven fabric or 
using as at least a portion of the wales, at least a portion of the 
courses or at least a portion of both the wales and the courses for a 
knitted fabric, the threads or tapes having the high shrinkability and 
high strength obtained above. The weaving or knitting can be carried out 
by using any conventional textile weaving or knitting technique. The 
specification of the starting threads or tapes used is optionally selected 
depending upon the shapes of articles to be packaged. For example, for a 
woven fabric when packaging cloth shrinkable only in an M-direction (not 
shrinkable in a T-direction) is necessary, the above-mentioned threads or 
tapes obtained from the heat shrinkable thermoplastic resins are used only 
as a warp and conventional threads or tapes having a low heat shrinkage 
factor can be used as a weft. On the other hand, when packaging cloth 
shrinkable only in a T-directions (not shrinkable in an M-direction) is 
necessary the weaving specification in which the above-mentioned warp and 
weft are inverted may be used. When a woven packaging cloth shrinkable 
both in the M- and T-directions is necessary, the above-mentioned threads 
or tapes of the heat shrinkable thermoplastic resins according to the 
present invention may be used as both warp and weft. 
Furthermore, when odd-shaped articles such as wheel discs of automobiles 
should be shrink-packaged, the desired woven shrink cloth can be obtained 
by using, as a warp, the above-mentioned threads or tapes having different 
heat shrinkage factors in accordance with the shapes of the articles 
(i.e., the warp having partially different shrinkage factors is used) and 
by using, as a weft, the threads or tapes having constant heat shrinkage 
factors. The heat shrinkage factors of the threads or tapes can be readily 
adjusted by changing the densities, melt flow rates, and blending ratios 
of the thermoplastic resins, the stretching temperatures, and the 
stretching ratios. 
The die usable in the practice of the present invention can be any 
conventional die, for example, T-dies, circular dies, filament type dies, 
or band type rectangular dies. The cooling after extrusion can be carried 
out by using, for example, water cooling, air cooling, or chill roll 
cooling. The stretching can be effected by any known technique, for 
example, oven stretching, roll stretching, wet type stretching, or heat 
plate type stretching. Since the difference between the melting point and 
the optimum stretching temperature of the above-mentioned thermoplastic 
resins according to the present invention is large, the use of the heat 
plate type stretching is especially preferable in the practice of the 
present invention, because the heat plate type stretching is relatively 
low cost and facilitates the heat control. 
Furthermore, the above-mentioned thermoplastic resins usable in the 
practice of the present invention can optionally contain any conventional 
additive, such as, antioxidants, ultraviolet light stabilizers, 
lubricants, pigments, and other resins, so long as the desired properties 
of the thermoplastic resins are not adversely affected. 
Lamination 
In another aspect of the present invention, there is provided a packaging 
cloth comprising a laminate containing a woven or knitted fabric derived 
from the above-mentioned threads or tapes. That is, the woven or knitted 
fabric obtained by using, as at least a portion of either or both of warp 
and weft for a woven fabric or at least a portion of either or both of the 
wales and courses for a knitted fabric, the thread or tape of the 
above-mentioned heat shrinkable thermoplastic resin is laminated onto both 
surfaces of at least one intermediate layer. The intermediated layer or 
layers are sandwiched by the two or more layers of the above-mentioned 
woven or knitted fabric. 
The intermediate layer can be prepared in any conventional manner from any 
thermoplastic resin, preferably having good adhesion properties to the 
woven or knitted fabric. Examples of such resin are: linear low-density 
ethylene polymers or low-density ethylene polymers having a branched long 
chain for the woven or knitted fabric composed of the above-mentioned 
linear low-density ethylene polymer or the mixtures thereof; and propylene 
polymers for the woven or knitted fabric composed of a minor amount of the 
above-mentioned mixture of the ethylene polymers and a major amount of 
propylene polymers. These intermediate layers can be prepared in any known 
manner and optionally may be subjected to, for example, an anchor 
treatment to improve the adhesion properties. Although there is no 
limitation in the thickness of the intermediate layer so long as the 
desired adhesion properties can be obtained, the preferable thickness is 
15 .mu.m or more. 
The use of the single woven or knitted packaging cloth mentioned 
hereinabove tends to cause the breakage of the shrink packaged cloth due 
to vibration during transportation in the case of heavy duty packaging. 
However, this tendency can be effectively improved by the use of the 
laminated packaging cloth according to the present invention. For example, 
when two layers of the above-mentioned packaging cloth are laminated, the 
dynamic load impact resistance against vibration breakage can be 
remarkably increased although the tensile strength increases only 2 times 
that of the single packaging cloth. That is, the dynamic load impact 
resistance test results are as follows: 
(a) Commercially available low-density polyethylene shrink film having a 
thickness of 150 .mu.m: 500 repeating times breakage, 
(b) Single shrinkable packaging cloth according to the present invention: 
2000 repeating times breakage, 
(c) Sandwiched laminating of two layers of the single shrinkable packaging 
cloth (b) according to the present invention: 15,000 repeating times 
breakage, 
(d) Sandwiched lamination of three layers of the single shrinkable 
packaging cloth (b) : no breakage by 30,000 repeating times. 
The intermediate layer having a relatively low melting point is preferably 
used in the practice of the present invention, due to the fact that the 
heat shrinkability of the laminated packaging cloth is decreased at the 
packaging if two or more layers of the woven or knitted cloth are directly 
heat fused. 
In a further aspect of the present invention, there is provided a packaging 
cloth comprising a laminate of the above-mentioned woven or knitted fabric 
onto one surface of a plastic film layer having a heat shrinkability 
identical or similar to that of the woven or knitted fabric layer. 
The above-mentioned plastic film layers are those preferably having heat 
shrinkage ratio such that the heat shrinkage ratio of one layer is 0.7 to 
1.3 times, more preferably 0.9 to 1.1 times, that of the other layer at a 
temperature between 80.degree. C. and 110.degree. C. Examples of such 
films are those obtained from low-density ethylene polymers, propylene 
polymers, polyvinyl chloride and having the desired heat shrinkage ratio 
by controlling the density, melt flow rates, the stretching temperature, 
and the stretching ratio. Although these is no specific limitation in the 
thickness of the film layer, the preferable thickness is 15 .mu.m or more. 
The lamination can be carried out in any conventional manner. For example, 
the heat shrinkable film can be laminated onto the woven or knitted cloth 
by using a conventional laminator. Although the similar resins are readily 
adhered to each other, the so-called anchor treatment can be 
advantageously used to improve the adhesion properties. Furthermore, three 
or more multilayers can be used in the preparation of the laminated 
packaging cloth according to the present invention. 
Thus, according to the present invention, the laminated shrink packaging 
cloth suitable for use in a heavy packaging field can be provided.