High density polyethylene type transparent film and process for production thereof

A 10 to 200 .mu.m thick film comprised of high density polyethylene having a density of 0.935 g/cm.sup.3 or more, the high density polyethylene type film having a film surface roughness of 0.001 to 0.15 .mu.m, a coefficient of C axis orientation of the cyrstals in the film (Fc) of 0.10 to 0.50, a difference .DELTA.n of the birefringence of the amorphous portions and crystalline portions in the film of 0.1000 to 0.1070, and a transparency of a haze of less than 10%. Such a film can be produced by passing an original material film through three heating rolls, designated as, for example, the first heating roll (hereinafter referred to as "R.sub.1 "), the second heating roll (hereinafter referred to as "R.sub.2 "), and the third heating roll (hereinafter referred to as "R.sub.3 "), in accordance with the order of the passage of the original material film, at a temperature of R.sub.2 higher than the temperatures of R.sub.1 and R.sub.3 and a temperature of R.sub.2 of 105.degree. C. or more below the melting point of the film, for heat treatment, then cooling the same. PA0 The obtained high density polyethylene type film has a superior transparency, a good balance of strength in the M direction and T direction, a remarkably large impact strength, and a large Young's modulus, and thus is suitable for use as a packing film for boxes for cigarettes, caramels, chocolates, etc., chewing gun, candy, and other confectionery.

TECHNICAL FIELD 
The present invention relates to a high density polyethylene type film with 
a remarkably superior transparency and a process for production thereof, 
more particularly, it relates to a transparent high density polyethylene 
type film with a good balance of strength in the M (machine) direction and 
T (transverse) direction, a remarkably high impact strength of the film, 
and a large Young's modulus, and a process for production thereof. 
The present invention further relates to a high density polyethylene type 
film having a high strength, superior transparency, and good heat 
sealability, and a process for production thereof. 
BACKGROUND ART 
In the past, to obtain a high density polyethylene (hereinafter referred to 
as "HDPE") transparent film, the general practice has been to pass a 
molten resin through a slit die and cool it rapidly by chill rolls or 
water. However, with this method, to improve the formability, use has been 
made of resins with low molecular weights. Therefore, the transparent film 
obtained has had a relatively weak strength. 
To obtain a strong film through the use of high molecular weight HDPE in 
forming a film, the general practice has been to use air-cooled inflation. 
With this method, it has been possible to obtain a film with superior 
strength through balancing the molecular orientation with the blow ratio, 
but since air is used for the cooling, only translucent or semitransparent 
films have been obtainable. 
Attempts have been made to achieve a certain degree of transparency (haze 
of about 15%) even with high molecular weight HDPE by passing the film 
through heated rolls having a surface gloss, so as to enhance the surface 
smoothness, but a transparency of a haze of 10% or less has not been 
obtained. 
To make an HDPE film transparent, it is known to uniaxially draw a 
semitransparent HDPE film to a ratio of 5 to 10 and press it between 
rolls. The film obtained by this method does indeed have a superior 
transparency, but features a remarkable orientation in the drawing 
direction or rolling direction, and thus there is remarkable 
directionality in the aspect of film strength, resulting in a tendency to 
easy tearing, the obtaining of an insufficient strength, an uneven heat 
contraction, warping in the film, heat sealing inability, and other 
defects. 
In the past, to obtain a high strength film from an HDPE film, the primary 
practice has been to use the inflation process. In inflation forming to 
obtain a high density film, as disclosed in Japanese Examined Patent 
Publication (Kokoku) No. 56-5172 and Japanese Unexamined Patent 
Publication (Kokai) No. 60-15122, it has been necessary to obtain a 
balance in the longitudinal and lateral orientation. 
That is, if the orientation in the longitudinal direction is too great, 
longitudinal tearing easily occurs and if the orientation in the lateral 
direction is too great, a high strength film can not be obtained. 
Therefore, in general, in inflation forming, consideration is given to the 
ratio of the inflation ratio of the bubble and the diameter of the die, 
i.e., the blow ratio, the take up speed, and the frost line height. 
However, although it has been possible to impart strength to a film with a 
balance in the longitudinal and lateral orientations, in the case of a 
high density polyethylene, the transparency is poor, and thus the 
applications are limited at the present stage. 
To improve the transparency of high density polyethylene type film, several 
other methods have been proposed. 
For example, Japanese Unexamined Patent Publication (Kokai) No. 53-31768 
proposes the technique of passing an original material thermoplastic resin 
film between a pair of rollers set to a clearance smaller than the 
thickness of the film and having a surface roughness of 0.5 s or less, at 
a temperature lower than the melting point or the softening point of the 
film, or lower, for rolling to obtain a film having a thickness smaller 
than the thickness of the original material film and a transparency of a 
haze of 4% or less. This technique is particularly characterized by the 
point of a maximum improvement of the surface state of the rolls used for 
the transparency treatment and, thereby, an improvement of the external 
haze, a factor governing the haze of film, to obtain a film with a 
superior transparency and gloss. However, by only defining the surface 
roughness of the rolls, improving the surface state of the rolls and, 
simultaneously, in this treatment, making the temperature lower than the 
melting point or the softening point of the film, or lower, and passing 
the film through the clearance between a pair of rolls, there are limits 
to the transparency of the HDPE film that can be obtained. In actuality, 
in this invention, examples relating to HDPE film were deleted from the 
embodiments in the subsequent examination process. 
On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 
59-5032, proposes a technique for treating by pressure an HDPE inflation 
original film between smooth surfaces under a constant temperature to 
improve the optical characteristics of the obtained film. However, this 
technique, like the above-mentioned technique, obtains a film with 
improved optical characteristics through an improvement of only one of the 
factors governing the haze of the film, i.e., the external haze, as shown 
by the statement in the publication that it was discovered at that time 
that the extent of the haze of the film and the extent of the 
insufficiency of the transparency were primarily due to the surface 
characteristics of the film. In the publication, there is a description of 
treating under pressure the original film between two or more opposingly 
positioned rollers and heating it under a constant temperature so as to 
facilitate the plastic deformation of the film, but there is no specific 
disclosure of heat treatment by the rolls. The embodiments only make 
specific mention of a method for placing the original film between two 
smooth sheets and using the contact with the sheet surfaces to flatten the 
original film. 
DISCLOSURE OF THE INVENTION 
The object of the present invention IS TO eliminate these problems in the 
prior art and to provide a transparent HDPE type film which is highly 
transparent, has a good balance in the film M direction and T direction, 
has a large film impact strength, has a high Young's modulus, and has 
excellent heat sealability, and a process for the production thereof. 
Other objects and novel features of the present invention will become clear 
from the disclosure of the overall description. 
According to the present invention, there is provided a 10 to 200 .mu.m 
thick film comprised of high density polyethylene having a density of 
0.935 g/cm.sup.3 or more, this high density polyethylene type film having 
a film surface roughness of 0.001 to 0.15 .mu.m, preferably 0.002 to 0.15 
.mu.m, more preferably 0.005 to 0.15 .mu.m, a coefficient of C axis 
orientation of the crystals in the film (Fe) of 0.10 to 0.50, preferably 
0.10 to 0.40, more preferably 0.10 to 0.30, a difference on of the 
birefringence of the amorphous portions and crystalline portions in the 
film of 0.1000 to 0.1070, preferably 0.1000 to 0.1065, more preferably 
0.1005 to 0.1060, and a transparency of a haze of less than 10%, 
preferably less than 8%. 
According to the present invention, there is further provided a 10 to 200 
.mu.m thick film comprised of 50% by weight or more of high density 
polyethylene having a density of 0.935 g/cm.sup.3 or more and high 
pressure low density polyethylene having a density of 0.910 to 0.930 
g/cm.sup.3 or straight chain low density polyethylene having a density of 
0.880 to 0.930 g/cm.sup.3, this high density polyethylene type film having 
a film surface roughness of 0.001 to 0.17 .mu.m, preferably 0.001 to 0.15 
.mu.m, more preferably 0.010 to 0.15 .mu.m, a coefficient of C axis 
orientation of the crystals in the film (Fc) of -0.3 to +0.50, preferably 
-0.2 to 0.40, more preferably -0.2 to 0.30, a difference .DELTA.n of the 
birefringence of the amorphous portions and crystalline portions in the 
film of 0.1000 to 0.1070, preferably 0.1000 to 0.1065, more preferably 
0.1005 to 0.1060, and a transparency of a haze of less than 10%, 
preferably less than 8%. Note that the amount of the above-mentioned high 
pressure low density polyethylene or straight chain low density 
polyethylene is preferably 1 to 50 parts by weight, particularly suitably 
1 to 30 parts by weight. 
Below, an explanation will be made of the case of use of three of these 
heating rolls. 
The present inventors engaged in in-depth studies of techniques for making 
a high density polyethylene transparent, whereupon they learned that, 
although it is necessary in the transparency treatment to pass the 
original material film between smooth surfaced rolls at a temperature 
under the melting point to obtain a thickness lower than the same, the 
transparency of the high density polyethylene is insufficiently improved 
by only passing the film through a pair of rolls at the same temperature, 
as in the prior art, and that by using three heating rolls, heating from 
the outside, providing the rolls with a temperature difference, and then 
rapidly cooling, a high density polyethylene can be obtained with a 
remarkably high transparency. It was learned that, according to the 
process, not only the roughness of the crystalline portions on the film 
surface, i.e., the external haze, but also the internal haze can be 
greatly reduced. Further, according to the present invention, and based on 
this discovery, there is provided a process for producing a high 
transparency high density polyethylene film having a haze of less than 10% 
characterized by passing an original material film comprised of a high 
density polyethylene having a density of 0.935 g/cm.sup.3 or more through 
three heating rolls under the below mentioned temperature conditions and 
then cooling the same. Note that, with regard to the temperature 
conditions of the above-mentioned heating rolls, designating the three 
rolls in the order of the passage of the original material film as the 
first heating roll (hereinafter referred to as "R.sub.1 "), the second 
heating roll (hereinafter referred to as "R.sub.2 "), and the third 
heating roll (hereinafter referred to as "R.sub.3 "), the temperature of 
R.sub.2 is set higher than the temperatures of R.sub.1 and R.sub.3 and the 
temperature of R.sub.2 is set at 105.degree. C. or more but below the 
melting point of the film. In particular, it is preferable to pass the 
original material film formed by inflation forming by a blow ratio of 3 or 
more under the above-mentioned temperature conditions for a draw ratio of 
3 or more. 
In the high density polyethylene type film of the first aspect of the 
present invention, an attempt is made to eliminate the above-mentioned 
problem points by the formation of a film of at least 50% by weight of 
high density polyethylene having a density of 0.935 g/cm.sup.3, preferably 
70% by weight or more, and a so-called high pressure low density 
polyethylene (hereinafter referred to as "LDPE") having a density of 0.910 
g/cm.sup.3 to 0.930 g/cm.sup.3. 
Further, in the high density polyethylene type film of the second aspect, 
an attempt is made to eliminate the above-mentioned problem points by 
forming a film from a composition comprised of at least 50% by weight of a 
high a density polyethylene having a density of 1 0.935 g/cm.sup.3, 
preferably 70% by weight or more, and a straight chain low density 
polyethylene copolymer (hereinafter referred to as "L-LDPE") having a 
density of 0.88 g/cm.sup.3 to 0.930 g/cm.sup.3, then heat treating the 
same by three or more heating rolls having a superior surface gloss. 
The high density polyethylene constituting the original material high 
density polyethylene film in the present invention may be an ethylene 
homopolymer or a copolymer of ethylene and one type or two types or more 
of comonomers. As an example of the copolymer involved, mention may be 
made of ethylene/propylene, ethylene/butene-1, and ethylene/hexene-1 
copolymers. Further, the high density polyethylene may be a blend with 
other polymers or may be a composition including antioxidants, dyes, 
inorganic fillers, and other additives. As examples of the other polymers 
to be blended therewith, mention may be made of low density polyethylene, 
polypropylene, copolymers of ethylene and vinyl acetate, and copolymers of 
ethylene and ethylene acrylate. At this time, the high density 
polyethylene is preferably included in an amount of at least 50%, 
preferably 70% or more. 
The original material film is produced by the inflation process, T-die 
process, and various other film-forming processes. In particular, good 
results can be obtained by applying the process of production of the 
present invention to a high density polyethylene film produced by the 
inflation process, where transparency is a problem. 
The density of the high density polyethylene is 0.935 g/cm.sup.3 or more, 
preferably 0.935 to 0.975 g/cm.sup.3, more preferably 0.945 to 0.960 
g/cm.sup.3. With a density of less than 0.935 g/cm.sup.3, it is impossible 
to provide superior properties to the HDPE film, e.g., nerve strength 
(Young's modulus), barrier property (moisture proofness), impact strength. 
When applying the present invention to a relatively high molecular weight 
polyethylene type resin or resin composition having the above density and 
seeking a greater strength in a film including at least 50% of a high 
density polyethylene, more preferably 70 to 90%, the melt index of the 
high density polyethylene (according to JIS K-7210, measurement under 
condition 4, hereinafter referred to as "MFR") should be made 1.0 g/10 min 
or less, preferably 0.5 g/10 min or less, more preferably 0.1 g/10 min or 
less, particularly preferably 0.06 g/10 min or less. The original material 
film is formed by the inflation process. 
The MFR of the LDPE and L-LDPE is usually 0.1 to 10.0 g/10 min, preferably 
0.1 to 3.0 g/10 min, particularly preferably 0.1 to 2.0 g/10 min. If an 
LDPE or L-LDPE with an MFR of less than 0.1 g is used, the fluidity is 
poor and thus the below mentioned formability of the film is poor, making 
production of an excellent film difficult. On the other hand, if an LDPE 
or L-LDPE with an MFR of over 10.0 g/10 min is used, a film having an 
excellent strength cannot be obtained. 
The blow ratio in the inflation molding is preferably 3 or more. When less 
than 3, the orientation in the lateral direction is small and the heat 
treatment of subsequent processes will result in an overly large 
orientation in the longitudinal direction, making it difficult to obtain a 
film with a sufficiently high strength. 
As the L-LDPE which can be used in the production of the film of the 
present invention, use may be made of one comprised of ethylene and 
.alpha.-olefin. As the .alpha.-olefin, use is suitably made of one having 
3 to 10 carbon atoms, for example, propylene, butene-1, hexene-1, octene, 
4-methyl-pentene. Further, the L-LDPE used is one with a density of 0.88 
g/cm.sup.3 to 0.930 g/cm.sup.3. Production of a straight chain low density 
PE copolymer having a density of less than 0.88 g/cm.sup.3 is extremely 
difficult. Further, if the density of the straight chain low PE copolymer 
is over 0.930 g/cm.sup.3, there is the inconvenience that the heat 
sealability cannot be improved. 
The film of the present invention can be formed from the above-mentioned 
high density PE and L-LDPE, but of course, antioxidants, dyes, etc., may 
be added in accordance with need. 
According to the film production process of the present invention, first, 
the above-mentioned high density PE or a composition comprised of L-LDPE 
or LDPE in the above-mentioned formulation proportion is formed into an 
original material film. As the method for forming the composition into a 
film, use may be made of the inflation process, T-die process, etc. When 
the above-mentioned composition is formed into a film by the inflation 
process, preferably the blow ratio is 3 or more. When the blow ratio is 
less than 3, the orientation in the lateral direction of the constituent 
molecules is insufficient and the orientation of the molecules in the 
longitudinal direction becomes too great due to the after-treatment 
accompanying the heat treatment, so there is the inconvenience that it is 
impossible to sufficiently improve the strength of the obtained film. 
The "transparency" of the film in the present invention may be divided into 
the transparency of the film surface and interior. The transparency of 
HDPE film formed by the air-cooled inflation process (for example, one 
with a thickness of 40 .mu.m [melt flow index 
(MI)=0.04 g/10 min, density (D)=0.949 g/cm.sup.3 ] was measured in terms of 
the surface factors and internal factors, and as a result, a total haze of 
the film (hereinafter also referred to as simply the "have") was, for 
example, 75%, which could be divided into a surface haze of 60% and an 
internal haze of 15%. In particular, the surface haze is derived from the 
scattering of light due to the roughness of the fine structure caused by 
crystallization of the film surface layer. Physical smoothing of the 
surface is possible in the present invention by making the surface 
roughness of the film 0.1 .mu.m or less, whereby a remarkable improvement 
is achieved. The principle is the same, for example, as with making ground 
glass (translucent) transparent by applying cellophane tape to its two 
sides. As an example of application, there is known the method of applying 
to the two sides of an HDPE film a resin with a good transparency, or 
extruding the same. However, as mentioned earlier, this method only 
reduces the scattering on the surface of the film, and with this along, 
sufficient transparency cannot be obtained. 
Therefore, unless the internal haze is eliminated, it is impossible to 
obtain a film with a superior transparency of haze of 15% or less in the 
above-mentioned case. 
Therefore, the present inventors engaged in thoroughgoing research into the 
factors causing the internal haze and considered that the factors of the 
internal haze were the sum of the fluctuation in the orientation of the 
crystal lattice axes (birefringence of a, b, and c axes of polyethylene) 
and the fluctuation in the birefringence arising from the difference 
.DELTA.n of the birefringence of the amorphous layer (portion) and 
crystalline layer (portion). They found the orientation of the crystal 
lattice axes from the coefficient of orientation Fc and .DELTA.n from 
Lorenz-Lorentz's formula and studied the coefficient of transparency of 
the film from various data, and as a result, they discovered that Fc and 
.DELTA.n have a close relationship with the transparency inside the film. 
That is, the greater the Fc (the more uniform the c axis orientation) and 
the smaller the .DELTA.n, the better the transparency inside the film. 
As a result, they discovered that, with a film thickness of 200 .mu.m or 
less, when Fc is 0.1 or more and .DELTA.n is 0.1070 or less, the 
transparency of an HDPE film with a smooth surface having a surface 
roughness of 0.1 .mu.m or less is remarkably improved, and thus completed 
the present invention. 
Here, the coefficient of orientation of the c axis Fc is found by the Stein 
method from the polarization infrared spectrum (Macromolecule 1, 116, 
1968): 
EQU Fa=(D730.sup.-1)/(D730+Z) 
EQU Fb=(D720.sup.-1)/(D720+Z) 
EQU Fa+Fb+Fc=0 
where, D730 and D720 are the infrared dichromatic ratios at 730 cm.sup.-1 
and 720 cm.sup.-1 and 
Fa, Fb, and Fc are the coefficients of orientation of the a, b, and c axis 
of the crystals. Further, the difference .DELTA.n of the birefringence of 
the crystal and amorphous portions is obtained from a modified 
Lorenz-Lorentz formula (Plastics, vol. 31, no. 2, p. 34) 
##EQU1## 
wherein, nD is the birefringence of the film and is measured by an Atsube 
type birefringence meter, .rho. is the film density and is found by a 
density gradient tube, and .DELTA..rho. is the difference in the density 
of the crystalline and amorphous portions. The values of the follow 
references were used. 
Crystalline density: 1.01 g/cm.sup.3, E. R. Walter, J. Polymer Sci 21, 
561.sup.c19 
Amorphous density: 0.85 g/cm.sup.3, A. K. Doolittle: J. App. Phys. 22, 
1471.sup.c19 The have measurement was in accordance with ASTM-D-1003. 
Further, the surface roughness, which indicates the smoothness of the film 
surface, was measured in accordance with the test method of JIS B0601-55. 
The resin in the present invention is designated as being a thermoplastic 
resin including at least 50% of HDPE having a density of 0.935 g/cm.sup.3 
or more, preferably 70% or more, in that the superior characteristics of 
an HDPE film, e.g., nerve strength (Young's modulus), barrier property 
(moisture proofness), impact strength, etc., are available with a density 
of 0.935 g/cm.sup.3 and further in that these characteristics can be 
secured even when blending in various additives or molecular weight 
increasers to modify the resin or blending resins, by including at least 
70% of the above-mentioned HDPE. 
Further, in the present invention, the film surface roughness is made 0.1 
.mu.m or less because there is great external roughness in the case of 
HDPE, which scatters light at the surface, and this is a factor behind the 
greater external haze. This is one of the conditions for elimination of 
this roughness and creation of the highly transparent film desired in the 
present invention. 
For the heating rolls, for example, use is made of metal rolls having a 
hard chrome plating on the surfaces thereof. These may be polished. 
Further, it is possible to use rolls having smooth surfaces which are 
worked or finished to a mirror-like gloss. 
Among the heating rolls, the temperature of the second roll (R.sub.2) must 
be made higher than that of the first roll (R.sub.1) and third roll 
(R.sub.3) That is, the original material film should be passed through 
heating rolls comprised of a system of R.sub.1, R.sub.2, and R.sub.3 with 
the intermediate roll set to the highest temperature and a temperature 
difference given among the rolls. 
The temperature of R.sub.1 is preferably 50.degree. C. or more, 55.degree. 
C. being particularly preferable. 
The temperature of the above-mentioned R.sub.2 must be 105.degree. C. or 
more but below the melting point of the original material film. Therefore, 
the heating rolls are all set to below the melting point of the original 
material film, but it is necessary to provide a temperature difference 
between R.sub.2 and the other rolls, i.e., R.sub.1 and R.sub.3. The 
temperature of R.sub.1 and R.sub.3 should be set to a temperature not 
higher than the temperature of R.sub.2. 
The temperature of R.sub.3 is preferably 70.degree. C. or more, 75.degree. 
C. or more being particularly preferable. If the temperature of R.sub.3 is 
over 120.degree. C., an excellent transparency is difficult to obtain. 
Further, at less than 70.degree. C., the film will adhere to R.sub.2 and a 
sufficient transparency would be difficult to obtain. 
In a preferred embodiment of the present invention, R.sub.2 is heated 
externally by an infrared heater, etc. The heating temperature is a 
temperature higher than 80.degree. C., but a temperature lower than the 
temperature at which the high density polyethylene would melt. Therefore, 
180.degree. C. or less (suitably 160.degree. C. or less) is preferable. 
The heating enables the acquisition of a film having sufficient uniform 
transparency even with an original material thinner than 50 .mu.m and a 
draw ratio of 1 to 3. By way of reference, if the original material film 
is thinner than 50 .mu.m, a film having a uniform transparency cannot be 
obtained without external heating. At this time, the heating is preferably 
applied uniformly to the surface which comes in contact with the roll in a 
manner such that the above-mentioned temperature range is achieved. 
The original material film may be passed through the clearance of the 
above-mentioned three heating rolls, which is less than the thickness of 
the original material film, to obtain a transparent film having a 
thickness less than that of the original material film. 
The draw ratio is made 3 or less. If drawing is greater than 3, the 
orientation in the longitudinal direction would progress too far and a 
film with a sufficient high strength could not be obtained. The preferable 
draw ratio or compression ratio is larger than 1 and no more than 3. 
The film which has passed through the heating rolls is then cooled. For 
example, it is cooled by two chill rolls (hereafter referred to as R.sub.4 
and R.sub.5). The temperature of the chill rolls is not particularly 
critical, but is preferably not more than 70.degree. C. and not less than 
30.degree. C. If over 70.degree. C. the chill rolls would not be able to 
perform their function. If less than 30.degree. C., it would be difficult 
to secure a sufficient flatness of the film. 
The thickness of the original material film used in the present invention 
is not particularly limited and is decided by the desired thickness of the 
product, etc. This is thicker than the thickness of the finished film, but 
less than 3 times the film thickness, preferably 2.3 times or less, more 
preferably 1.05 to 1.8 times the film thickness. 
According to the present invention, a high transparent high density 
polyethylene having a haze less than 10% is obtained. The haze is measured 
according to ASTM D-1003. The haze in the present invention refers to the 
sum of the external haze and the internal haze. 
The high density polyethylene film crystallizes at the surface in the free 
surface state after film formation by the inflation process, and lamera 
aggregates (l=100 to 110 .ANG.) protrude as roughness from the surface. 
The magnitude of the same corresponds to the wavelength (4000 to 8000 
.ANG.) of visible light, so scattering occurs and the film becomes 
nontransparent. 
Due to the transparency treatment of the present invention, the surface 
roughness is smoothed in a state of easy movement of molecular chains, the 
wavelength of the roughness is 4000 .ANG. or less, and the external haze 
is made excellent. The present invention has the important feature of 
enabling a reduction of not only the external haze, but also the internal 
haze. As factors of nontransparency from inside films, there are 
considered the nonuniform thickness of microcrystals and the internal 
voids included in the crystals. 
In the present invention, due to the transparency treatment, for example, a 
density of of a film before treatment of 0.948 g/cm.sup.3 is raised to a 
density of a film after treatment of 0.952 g/cm.sup.3, and it is 
considered that the above-mentioned internal voids and nonuniform layer 
are eliminated.

EXAMPLES 
The present invention will now be explained by the following Examples and 
Comparative Examples. 
Examples 1 to 3 and Comparative Examples 1 to 4 
In the Examples and Comparative Examples, HDPE (D =0.949 g/cm.sup.3, MI 
=0.04 g/10 min) was subjected to the inflation process to produce an 
original material film 50 .mu.m in thickness. The original material film 
was passed through heating rolls having a surface gloss (temperature below 
melting point of resin) to produce a film with a surface roughness of 0.1 
.mu.m or less and different Fc's and .DELTA.n's. The physical properties 
of the films are shown in Table 1. 
However, in Example 3 and Comparative Example 4, use was made of a blend of 
HDPE and low density polyethylene (LDPE, D=0.921, MI=1.5). Otherwise, the 
procedures were the same as above. 
(1) Forming material: 65.phi. Ext, die: 100.phi. spiral 
(2) Temperature C.sub.1 =C.sub.2 =C.sub.3 =H=D=190.degree. C. 
(3) Take up speed: 15 m/min 
(4) Blow ratio (BVR)=4.0 
(5) Film thickness: 50 .mu.m 
TABLE 1 
__________________________________________________________________________ 
Example Comparative Example 
1 2 3 4 1 2 3 4 
HDPE HDPE HDPE 
(80%) 
(80%) (60%) 
LDPE L-LDPE LDPE 
Resin used 
HDPE 
HDPE 
(20%) 
(20%) HDPE HDPE HDPE (40%) 
__________________________________________________________________________ 
Fc 0.201 
0.315 
0.150 
0.160 0.023 
0.023 
0.195 
0.050 
.DELTA.n 0.1057 
0.1048 
0.1035 
0.1040 
0.1075 
0.1063 
0.1077 
0.1060 
Film properties 
Haze (%) 9.5 3.5 4.2 -3.8 28 23 19 15 
Impact 280 250 20 285 260 290 250 180 
(kg .multidot. cm/mm) 
Young's 9,800 
10,500 
7,200 
7,800 8,800 
8,500 
8,300 
5,500 
modulus 
Roughness (.mu.m) 
0.108 
0.075 
0.090 
0.085 -- 0.205 
-- 0.175 
(Film thickness 45 .mu.m) 
__________________________________________________________________________ 
L-LDPE = density 0.917 g/cm.sup.3, MFR = 0.79 g/10 min, ethyl groups/100 
= 20 ethylenebutene-1 copolymer. 
Example 4 
Use was made of an HDPE having a melt index of 0.05 g/10 min and a density 
of 0.949 g/cm.sup.3. Using the inflation process, a blow ratio of 5, a 
frost line of 500 mm, and a take up speed of 10 m/min, a 100 .mu.m thick 
original material film was formed. This was drawn at a draw ratio of 3 and 
heat treated under the conditions shown in Table 1. Note that the chill 
roll temperatures were R.sub.4 =R.sub.5 :50.degree. C. 
Note that in the following examples, the breaking strength was determined 
according to JIS-Z-1702. 
Examples 5 to 9 and Comparative Examples 5 to 7 
High density polyethylene film was obtained in the same manner except for 
using the conditions shown in Table 2. The results are shown in Table 2. 
Comparative Example 8 
High density polyethylene film was obtained in the same manner as in 
Example 2, except that the blow ratio was made 2 and the conditions shown 
in Table 2 were used. 
The results are shown in Table 2. 
TABLE 2 
__________________________________________________________________________ 
R.sub.1 R.sub.2 
R.sub.3 
Haze 
Breaking strength 
Thickness after 
(.degree.C.) 
(.degree.C.) 
(.degree.C.) 
(%) 
(kg/cm.sup.2) MD/TD 
treatment (.mu.m) 
__________________________________________________________________________ 
Ex. 4 70 128 85 5.5 
445/415 82 
Ex. 5 105 128 105 5.2 
475/398 67 
Ex. 6 105 120 105 5.6 
460/408 73 
Ex. 7 60 120 85 8.8 
453/438 87 
Ex. 8 110 120 85 7.1 
488/385 68 
Ex. 9 70 121 85 7.3 
461/418 84 
Com. Ex. 5 
70 138 85 Stuck on 2nd roll 
Com. Ex. 6 
70 125 130 58 453/395 72 
Com. Ex. 7 
70 90 85 68 473/435 68 
Com. Ex. 8 
70 125 105 6.3 
553/223 65 
__________________________________________________________________________ 
Examples 10 to 12 
High density polyethylene (density 0.950 g/cm.sup.3, blow ratio 3) with an 
MFR of 0.05 g/10 min and a melting point of 130.degree. C. was subjected 
to a usual inflation process to produce original materials having 
thicknesses of 20 .mu.m, 30 .mu.m, and 40 .mu.m (hereinafter referred to 
as "original material (1)", "original material (2)", and "original 
material (3)"). These original materials (shown in Table 3) were used to 
produce films with thicknesses as shown in Table 4, using rolls with a 
superior surface gloss at an R.sub.1 temperature of 85.degree. C., an 
R.sub.2 temperature of 115.degree. C., an R.sub.3 temperature of 
110.degree. C., and a chill roll R.sub.4 and R.sub.5 temperature of 
50.degree. C. The haze of the obtained films was measured, and the results 
are shown in Table 3. Note that R.sub.2 was heated by an infrared heater. 
TABLE 3 
______________________________________ 
Film Thick- Haze 
Example Orig. mat. type 
ness (.mu.m) 
value (%) 
______________________________________ 
Ex. 10 Orig. mat. (1) 
15 2.5 
Ex. 11 Orig. mat. (2) 
21 2.8 
Ex. 12 Orig. mat. (3) 
50 4.5 
Com. Ex. 9 
Orig. mat. (1) 
15 -- 
Com. Ex. 10 
Orig. mat. (2) 
21 -- 
Com. Ex. 11 
Orig. mat. (3) 
30 -- 
______________________________________ 
Note that the films obtained by Examples 10 to 12 were all uniformly 
transparent. Note further that Comparative Examples 9 to 11 did not make 
use of an infrared heater, but used heating rolls for the heating. In 
Comparative Examples 9 to 11, films having a uniform transparency could 
not be obtained. 
Examples 13 to 14 and Comparative Examples 12 to 14 
For the high density PE, use was made of one having a density of 0.950 
g/cm.sup.3 and a melt index of 0.04 g/10 min. For the straight chain low 
density PE copolymer, use was made of one with a density of 0.88 
g/cm.sup.3 and a melt index of 4 g/10 min. 
The formulation ratio of high density PE and straight chain low density PE 
copolymer was 100/0 in Comparative Example 12, 95/5 in Comparative Example 
13, 70/30 in Example 13, 60/40 in Example 14, and 40/60 in Comparative 
Example 14. 
The compositions comprised of the above formulations were subjected to the 
inflation process to make films having thicknesses of 100 .mu.m, which 
were used as the original material films. At this time, the forming 
temperature was 200.degree. C. and the blow ratio 3. 
The original material films produced as above were heat treated by the 
three heating rolls, and were then cooled by the two chill rolls to obtain 
films 70 .mu.m in thickness. The temperatures of the heating rolls were 
set to 100.degree. C. for the first roll (R.sub.1), 115.degree. C. for the 
second roll (R.sub.2), and 100.degree. C. for the third roll (R.sub.3). 
Further, the temperature of the chill rolls was set to 30.degree. C. 
The physical properties of the films thus obtained are shown in Table 4. 
TABLE 4 
______________________________________ 
Low 
temp. 
Haze Yield Young's seal- Impact 
value strength modulus ability 
strength 
(%) (kg/cm.sup.2) 
(kg/cm.sup.2) 
(.degree.C.) 
(kg .multidot. cm/mm) 
______________________________________ 
Com. Ex. 
6 2 10,500 135 188 
12 
Com. Ex. 
6.0 1.9 9,300 133 210 
13 
Ex. 13 4.3 1.75 8,700 120 320 
Ex. 14 4.5 1.35 7,600 115 315 
Com. Ex. 
4.8 0.98 5,100 105 285 
14 
______________________________________ 
The physical properties were determined by the following methods: 
Haze . . . according to ASTM D1003 
Yield strength . . . according to JIS Z1702 
Young's modulus . . . according to ASTM D882 
Heat sealability . . . First, the film was cut into narrow strips 15 mm 
wide. These were heat sealed under conditions of a sealing pressure of 2 
kg/cm.sup.2 and a sealing time of 1 second, with different temperatures, 
then the test pieces were peeled at a speed of 300 mm/min to determine the 
peeling strength. The heat sealability was expressed by the sealing 
temperature of test pieces displaying a peeling strength of 1 kg. 
Impact strength . . . according to ASTM-D-781 
From the results of Table 4, it was confirmed that the film of the present 
invention had superior impact strength and a good balance of such physical 
properties as the heat sealability, haze, yield strength, and Young's 
modulus. 
EFFECTS OF THE INVENTION 
According to the present invention, as indicated also by the 
above-mentioned embodiments, a high density polyethylene type film with a 
small haze and superior transparency is obtained. This film further has 
both transparency and film strength, so although conventional high density 
polyethylene films were considered difficult to make highly transparent 
without obstructing the film strength, this was realized in the present 
invention. Therefore, the industrial significance is great. The high 
density polyethylene type film of the present invention, having the 
constitution explained above, has flexability and a suitable melting 
temperature, so in addition to the superior characteristics of high 
density polyethylene, i.e., high strength and high transparency, it is 
provided with an excellent impact resistance and heat sealability. 
Therefore, the film of the first aspect of the invention can be easily 
heat sealed at a low temperature and is difficult to break even under 
impact. Further, according to the process of production of the present 
invention, it is possible to produce a film having not only a high 
strength, easy heat sealability, and high impact strength, but also an 
excellent transparency.