Bag having excellent blocking resistance

Bags having increased blocking resistance properties are formed of a film composed of a resin blend of a polyethylene (PE) resin and a polybutylene terephthalate (PBT) resin having an intrinsic viscosity or 0.7 of more. Preferably, the film is a multi-layer structure formed via coextrusion inflation molding techniques having inner and outer layers laminated to one another. The inner layer is a polyolefin, while the outer layer is composed of a PE/PBT blend, in which the PBT is present in an amount of between 5 to 30 wt. % based on the total weight of the resin composition and has a melt index of 0.7 or more. The thickness ratio of the internal layer to the external layer is preferably between 95/5 to 30/70.

FIELD OF THE INVENTION 
The present invention relates to a synthetic resin bag having excellent 
blocking resistance. 
BACKGROUND AND SUMMARY OF THE INVENTION 
Conventional bags which are adapted to be filled with fertilizers, feed, 
chemicals, and the like (i.e., the so-called "heavy-load bags") are 
typically made from polyethylene resin due to its physical strength and 
low cost (particularly when such bags are formed from a tubular 
polyethylene film by inflation molding techniques). 
However, when heavy-load bags made of a polyethylene resin are filled with 
high temperature materials (e.g. between about 80 to 100.degree. C.) and 
placed one on top of the other, thermal adhesion between bags in contact 
with one another usually occurs. This thermal adhesion (known as 
"blocking" in art parlance) not only makes the bags more difficult to 
handle, but also increases the possibility of bag breakage. For this 
reason, high temperature materials are usually either cooled below about 
60.degree. C before the heavy-load bags are filled, or the filled bags are 
cooled before being placed upon a pallet. 
Examples of thermoplastic resins which are known to exhibit excellent heat 
resistance properties include polyethylene terephthalate resins, polyamide 
resins, polyphenylene sulfide resins, and polycarbonate resins. However, 
these resins are impractical to use for heavy-load bags due to the resins' 
high costs, difficult film-forming properties (particularly by inflation 
molding), and poor productivity during bag formation. Although blending 
such a heat-resistant resin with polyethylene resin has also been 
considered as an expedient, such a proposal has not been put to practical 
use due to the wide variance in mechanical and thermal properties that 
occur depending upon the dispersion and kneading conditions employed to 
form these resin blends. 
Therefore, it is an object of the present invention to provide a bag having 
excellent blocking resistance which would thus be especially well suited 
for holding high-temperature materials. According to the invention, it has 
been found that blocking resistance for a bag can be increased by 
10.degree. to 30.degree. C. when the bag includes an extruded external 
film layer comprised of between about 5 to 30% by weight (based on the 
total weight of a composition) of a particular polyethylene resin (to be 
described below). 
The term "blocking resistance" used herein and in the accompanying claims 
is intended to mean a value obtained by measurement according to JIS Z 
1514, items 6, 7 (incorporated by reference herein). 
Accordingly, the present invention provides a bag having excellent blocking 
resistance characterized by including an external film layer comprised of 
a resin composition composed of a polyethylene (PE) resin and between 
about 5 to 30% by weight (based on the resin composition) of a 
polybutylene terephthalate (PBT) resin having an intrinsic viscosity of 
0.7 or more. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Any suitable polyethylene resin may be used according to the present 
invention so as to form a mixture with polybutylene terephthalate resin. 
In this regard, any polyethylene resin usually employed in forming bags 
may be used. Examples of the suitable polyethylene resins include 
low-density polyethylene resins (LDPE), linear low-density polyethylene 
resins (L-LDPE), and copolymers thereof, which may be used either alone or 
in combination. L-LDPE is particularly preferable. The melt index of the 
polyethylene resin is preferably between 0.1 to 5.0. 
The polybutylene terephthalate resin that may be used to form the bags of 
the present invention is preferably one formed by polycondensation of 
1,4-butanediol with terephthalic acid, or an ester thereof, with a lower 
alcohol and may be a copolymer mainly composed of polybutylene 
terephthalate. In the present invention, it is necessary to use 
polybutylene terephthalate having an intrinsic viscosity of 0.7 or greater 
from the viewpoint of preparing a film by inflation molding techniques. 
Use of a polybutylene terephthalate resin having an intrinsic viscosity of 
1.0 or greater is particularly preferred. 
The present invention is characterized by using as an external layer of a 
bag, a film prepared from a resin composition composed of the 
above-described polyethylene resin and between 5 to 30% by weight (based 
on the total weight of the composition) of a polybutylene terephthalate 
resin blended with the polyethylene resin. When the polybutylene 
terephthalate resin content of the film is less than 5% by weight, the 
blocking resistance is not noticeably improved. On the other hand, when 
the content exceeds 30% by weight, the dispersion of the polybutylene 
terephthalate resin in the polyethylene resin is poor. Poor PBT dispersion 
unfavorably lowers the composition's physical and film-forming properties, 
and also lowers the adhesion between the internal and external film layers 
which may form the bag. 
The polyethylene resin may be previously blended with the polybutylene 
terephthalate resin in an extruder separate from the film-forming line, or 
alternatively, may be blended in an extruder just prior to film formation. 
However, since physical properties are reduced when polybutylene 
terephthalate resin dispersion is poor, it is preferable to blend the PE 
and PBT previously in a separate extruder. 
Although it is possible to form a bag having desired blocking resistance 
using a single layer film formed of the above-described PE/PBT resin 
blend, the bag may notexhibit all of the physical properties (e.g. tear 
strength) desired and/or required for some end-use applications . 
Accordingly, it is particularly preferred to form the bags of the present 
invention from a film laminate. The film laminate employed to form the 
bags of this invention will include an outer film layer comprised of the 
PE/PBT blend discussed above, laminated to an inner polyolefin film layer. 
A bag formed of such a laminated film will exhibit a desirable combination 
of mechanical strength and blocking resistance. 
The laminated film embodiment of the present invention thus provides a bag 
having excellent blocking resistance, and is formed using a film prepared 
by molding and laminating a polyolefin resin by inflation co-extrusion on 
the interior of a film formed of a resin composition which includes a 
polyethylene resin and between 5 to 30% by weight (based on the resin 
composition) of a polybutylene terephthalate resin having an intrinsic 
viscosity of 0.7 or more. 
The polyethylene resin which may be used in admixture with polybutylene 
terephthalate resin is substantially the same as that used in the first 
embodiment of the invention described above (i.e., the non-laminated film 
embodiment). However, in the laminated film embodiment of the invention it 
is preferred that the melt index of the polyethylene resin be from 0.1 to 
15. When the melt index is less than 0.1, the extruder motor load during 
film formation due to high resin viscosity is high, thereby lowering 
productivity. On the other hand, when the melt index exceeds 15, the 
viscosity is too low such that productivity is similarly lowered in 
addition to making it difficult (and usually impossible) to form a 
homogeneous coextruded film. The melt index of the polyethylene is most 
preferably between 0.1 to 5.0. 
Examples of suitable polyolefin resins that may be used for the internal 
film layer according to the present invention include homopolymers of 
olefins, such as ethylene, propylene, butene, isoprene, pentene and 
methylpentene, and their copolymers, having a melt index of between 0.1 to 
5.0. Polyethylene and its copolymers are particularly preferred. When the 
melt index is less than 0.1, the motor load on the extruder during film 
formation increases due to high viscosity, which lowers productivity. On 
the other hand, when the melt index exceeds 5.0, the viscosity is too low, 
which not only makes film formation difficult, but also unfavorably lowers 
the mechanical properties (particularly tensile elongation) of the film. 
It is impossible to successfully laminate the polyolefin resin film 
(internal layer) onto a polybutylene terephthalate-containing polyethylene 
resin film (external layer) through thermal adhesion. Moreover, use of an 
adhesive for lamination also often results in delamination during use. For 
this reason, it is preferred to form the bags of this invention from a 
film having a two-layer film structure prepared by inflation co-extrusion 
using polyolefin resin as an internal layer, and polyethylene resin 
composition containing between 5 to 30% by weight (based on the total 
weight of the composition) of polybutylene terephthalate resin as an 
external layer. It is therefore possible to form a bag exhibiting an 
increased blocking resistance of between about 5 to 30.degree. C., 
sufficient interlaminar adhesion strength without the use of an adhesive, 
and strengths necessary for a heavy-load bag (particularly tensile 
elongation and tear strength in a direction perpendicular to the extrusion 
direction of the film) comparable to those of a bag formed entirely of 
polyethylene resin. 
In the case of a multi-layer film laminate structure, the external layer is 
preferably as thin as possible since it is used for the purpose of 
improving the bag's blocking resistance. The internal layer, however, has 
a thickness sufficient to provide the necessary mechanical strength for 
the bag. The total thickness of the film laminate is preferably between 50 
to 500.mu.m, and more preferably between 60 to 300.mu.m. Moreover, the 
thickness ratio of the internal layer to the external layer is preferably 
between 95/5 to 30/70. 
In the case of a resin composition comprising a polyethylene resin having a 
melt index of 0.1 to 15 which includes between 5 to 30% by weight (based 
on the total weight of the composition) of a polybutylene terephthalate 
resin having an intrinsic viscosity of 0.7 or more, it is preferred that 
the viscosity ratio of the polybutylene terephthalate resin to the 
polyethylene resin be within the range represented by the following 
formula: 
viscosity ratio .mu..sub.A /.mu..sub.B =0.15 to 2.5 
wherein 
.mu..sub.A : viscosity of polybutylene terephthalate resin as determined at 
a shear rate of 100 sec.sup.-1 at 240.degree. C., 
.mu..sub.B : viscosity of polyethylene resin as determined at a shear rate 
of 100 sec.sup.-1 at 240.degree. C. 
When the viscosity ratio, .mu..sub.A /.mu..sub.B, is less than 0.15 or 
exceeds 2.5, the resin dispersion is poor. 
Known additives for thermoplastic and thermosetting resins, i.e., 
plasticizers, stabilizers such as antioxidants and ultraviolet absorbers, 
antistatic agents, surfactants, colorants such as dyes and pigments, 
lubricating agents for the purpose of improving flowability, 
crystallization accelerators (nucleating agents), etc. may be added to the 
resin compositions described above to achieve desired performance 
characteristics. It is also possible to add as auxiliaries small amounts 
of other thermoplastic resins and inorganic fillers as may be needed to 
achieve certain purposes as long as they do not deleteriously affect the 
present invention. 
The bags of the invention are most preferably formed of a composition 
comprising between 5 to 30 wt. % of polybutylene terephthalate having an 
intrinsic viscosity of 0.7 or larger and 70 to 95 wt. % of polyethylene 
(based on the weight of the composition). 
The bags of the present invention thus exhibit improved blocking resistance 
without sacrificing the mechanical strength necessary for such bags, e.g., 
tensile strength, tensile elongation and tear strength. The increased 
blocking resistance also allows the cooling time for high-temperature 
materials intended to be placed in the bags to be reduced (or eliminated).

EXAMPLES 
The present invention will now be described in more detail with reference 
to the following Examples, which should not be construed as limiting the 
scope of the present invention. 
EXAMPLES 1 TO 8 AND COMATIVE EXAMPLES 1 TO 6 
Films shown in Table 1 were prepared by the following method and subjected 
to physical property evaluation as described below. 
A linear low-density polyethylene resin having a melt index of 0.5 (Ultzex 
2005; a product of Mitsui Petrochemical Industries, Ltd.) used in prior 
art heavy-load bags was kneaded with a polybutylene terephthalate resin 
having an intrinsic viscosity of 1.4 (as determined in o-chlorophenol 
solution at 25.degree. C.) with a twin-screw extruder of 40 mm.phi. at a 
cylinder temperature of 240.degree. C. in amounts of blending shown in 
Table 1. The mixture was thereafter pelletized. The viscosity (.mu..sub.A) 
of the polybutylene terephthalate resin as determined at a shear rate of 
100 sec .sup.-1 at 240.degree. C. and the viscosity (.mu..sub.B) of the 
polyethylene resin as determined at a shear rate of 100 sec.sup.-1 at 
240.degree. C. were 15000P and 12000P, respectively, so that the viscosity 
ratio .mu..sub.A /.mu..sub.B was 1.25. 
The kneaded resin was extruded with an inflation molding machine through a 
die having a diameter of 100 mm.phi. at a cylinder temperature of 
220.degree. C. and a die temperature of 240.degree. C. to prepare an 
inflation film having a blow-up ratio of 2.9. Then, extrusion was 
conducted with a two-layer inflation film molding machine through a die 
having a diameter of 100 mm.phi. by making use of the kneaded resin as an 
external layer and a linear low-density polyethylene resin having a melt 
index of 0.5 as an internal layer at an internal layer extruder cylinder 
temperature of 220.degree. C. and a die temperature of 240.degree. C. to 
prepare an inflation film having a blow-up ratio of 2.9 and a thickness of 
150 .mu.m (internal layer/external layer=50/50). 
The data on Comparative Example 1 are physical properties of a simple 
inflation film comprising only a linear low-density polyethylene resin 
having a melt index of 0.5. 
The physical properties of the bag were evaluated by the following 
measuring method. 
Blockinq resistance 
Blocking resistance properties were measured according to JIS Z 1514. 
Heat-sealability 
A test piece of a width of 15mm was sealed under heat sealing conditions of 
a temperature of 80.degree. C., a pressure of 1 kg/cm.sup.2 and a time of 
2 sec, and sealing strengths of 0 to 500 gf, 500 to 1000 gf, 1000 to 1500 
gf and 1500 gf or more were evaluated as X, .DELTA., .largecircle., and 
.circleincircle., respectively. 
Interlaminar bonding strength 
The bonding strength between the external layer and the internal layer was 
measured according to JIS 6854 and evaluated as follows: 
.circleincircle.: bonding strength was very high, and no peeling occurred. 
.largecircle.: bonding strength was high, and peeling hardly occurred. 
.DELTA.: bonding strength was very low, and peeling readily occurred. 
x: bonding strength was very low, and peeling readily occurred. 
Melt Index 
Melt Index was measured at 190.degree. C. under a load of 2160 g according 
to ASTM D 1238. 
While the invention has been described in connection with what is presently 
considered to be the most practical and preferred embodiment, it is to be 
understood that the invention is not to be limited to the disclosed 
embodiment, but on the contrary, is intended to cover various 
modifications and equivalent arrangements included within the spirit and 
scope of the appended claims. 
TABLE 1 
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Comp. 
Comp. Comp. 
Comp. 
Ex. 1 
Ex. 2 
Ex. 1 
Ex. 2 
Ex. 3 
Ex. 4 
Ex. 3 
Ex. 4 
__________________________________________________________________________ 
PBT resin in external 
0 3 5 10 15 30 50 100 
layer (wt %) (single- 
(single- 
(single- 
(single- 
(single- 
(single- 
(single- 
layer) 
layer) 
layer) 
layer) 
layer) 
layer) 
layer) 
film thickness 
external 
150 150 150 150 150 150 150 150 
(.mu.m) layer 
internal 
-- -- -- -- -- -- -- -- 
layer 
tensile strength 
TD 108 107 110 110 88 61 33 450 
(yield point) 
(kgf/cm.sup.2) 
tensile strength 
TD 500 or 
300 200 150 80 15 3 400 
(%) more 
tear strength 
MD 71 49 48 6 6 5 3 10 
(kgf/cm) TD 207 206 211 219 161 113 72 12 
JIS Z 1702 
interlaminar bonding strength 
-- -- -- -- -- -- -- -- 
heat-sealability 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.largecircle. 
.largecircle. 
.DELTA. 
X X 
blocking resistance (.degree.C.) 
55 60 65 80 85 90 105 210 
__________________________________________________________________________ 
Comp. Comp. 
Ex. 5 
Ex. 5 
Ex. 6 
Ex. 7 
Ex. 8 
Ex. 6 
__________________________________________________________________________ 
PBT resin in external 
3 5 10 15 30 50 
layer (wt %) 
film thickness 
external 
75 75 75 75 75 75 
(.mu.m) layer 
internal 
75 75 75 75 75 75 
layer 
tensile strength 
TD 103 103 107 103 108 109 
(yield point) 
(kgf/cm.sup.2) 
tensile strength 
TD 500 or 
500 or 
500 or 
500 or 
500 or 
500 or 
(%) more 
more 
more 
more 
more 
more 
tear strength 
MD 81 77 70 70 64 41 
(kgf/cm) TD 210 210 215 203 189 166 
JIS Z 1702 
interlaminar bonding strength 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.DELTA. 
heat-sealability 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
blocking resistance (.degree.C.) 
60 65 80 85 90 105 
__________________________________________________________________________ 
Note: 
MD: direction of flow during film formation 
TD: direction perpendicular to MD