Method for the preparation of a crystalline thermoplastic resin sheet

The invention provides a method for the preparation of a sheet of a crystalline thermoplastic resin, e.g. polypropylene, having excellent transparency and surface properties as well as thermoformability. The inventive method comprises quenching of a molten resin sheet extruded out of a T-die by introducing the same into a slit where cooling water is flowing at a velocity higher than the running velocity of the extruded molten resin sheet and then stretching the thus quenched resin sheet in a stretch ratio in the range from 1.02 to 1.5 times.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for the preparation of a 
crystalline thermoplastic resin sheet or, more particularly, to a method 
for the stable and high-speed preparation of a crystalline resin sheet 
having excellent transparency, high gloss and uniformity which can be used 
as such for the fabrication of a paper folder and the like cardboard 
products or for the fabrication of various boxes and containers utilizing 
the excellent thermoformability thereof. 
Conventional thermoplastic resin sheets of high transparency used hitherto 
are mostly made of a non-crystalline or amorphous resin such as a 
polyvinyl chloride resin, polystyrene-based resin and the like. Sheets of 
these amorphous thermoplastic resins have excellent thermoformability so 
that they are widely used in the fabrication of various boxes and 
containers useful in many fields. Among these amorphous thermoplastic 
resins, however, polyvinyl chloride resins are disadvantagenous in 
respects of hygiene, heat resistance, moisture proofness and other 
properties. Moreover, wastes of the resin cannot be disposed without the 
very serious problem of environmental pollution, for example, due to the 
emission of chlorine-containing gases in the course of incineration for 
disposal. Polystyrene-based resins are also not quite satisfactory in 
respects of the heat resistance, impact strength, moisture-proofness and 
other properties. Notwithstanding these disadvantages and problems, sheets 
of these amorphous thermoplastic resins are widely used as a packaging 
material in many fields solely by virtue of their excellent transparency. 
Accordingly, it is a recent trend to convert the base resin of the sheets 
from the above mentioned amorphous thermoplastic resins to a crystalline 
resin having excellent mechanical strength and heat resistance. 
Crystalline thermoplastic resins are, of course, inferior in the 
transparency due to the crystallinity. Although the transparency of a 
crystalline thermoplastic resin sheet can be increased by quenching in 
cold water, appearance of haze dots is unavoidable in the sheet which has 
been quenched so that such a quenched sheet is no longer acceptable with 
its poor appearance to be utilized as such and difficulties are also 
encountered when such a sheet is to be used as a packaging material in the 
form of a box or other container. 
Nevertheless, polypropylene-based resins, for example, are increasingly 
used in recent years in place of the polyvinyl chloride resins and 
polystyrene-based resins by virtue of their excellent strength, rigidity, 
heat resistance, moisture proofness and other properties. They are, 
however, far from satisfactory when transparency is an essential 
characteristic of the resin sheet and the applicability of these resins is 
greatly limited also owing to their relatively low rigidity and 
thermoformability in comparison with the above mentioned amorphous 
thermoplastic resins. In particular, the low transparency so far thereof 
is the determinant factor for the absence of competitiveness with the 
amorphous resins. 
Various attempts and proposals have been made hitherto to improve the 
transparency of a polypropylene sheet. In connection with the quenching 
method of an extruded sheet of a molten resin to control the crystalline 
structure thereof, for example, following methods have been proposed. 
The first is the chill roll method which is, however, not suitable when 
quenching is desired down to a temperature below the dew point because dew 
drops are deposited on the surface of the chill roll which is at a 
temperature below the dew point. In addition to the above mentioned 
limitation in respect of quenching, no resin sheet of excellent surface 
condition can be obtained by this method because air is unavoidably caught 
between the roll surface and the running molten resin sheet, especially, 
in high-speed molding. 
The second of the proposals is the water quenching method. Although a high 
quenching effect can be obtained in a conventional water quenching method, 
the difficulty in obtaining uniformity in the quenching effect frequently 
results in the appearance of boiling dots and haze dots in the quenched 
resin sheet. Therefore, sufficiently uniform resin sheets can be obtained 
only at an extremely low molding velocity. 
Thirdly, the method of slit water quenching has been proposed which, 
although the method is advantageous in respect of the efficiency of 
quenching and uniformity of quenching, is not always quite satisfactory 
because the method is not suitable for highspeed molding and not free from 
the problems of haze dots and stripes, unevenness in the thickness of the 
sheet, macroscopic undulation on the surface of the sheet and curling of 
the sheet. 
Furthermore, following methods have been proposed for the improvement of 
the transparency of polypropylene resin sheets. 
Thus, the fourth method is the use of a resin admixed with a nucleating 
agent but this method is not free from the problems of bleeding of the 
nucleating agent on the surface of the sheet as well as the unpleasant or 
offensive odor and toxicity thereof in addition to the limited improvement 
of the transparency. 
The fifth method is the use of a resin blended with a petroleum resin. This 
method is, however, disadvantageous due to the decrease in the heat 
resistance and moisture proofness inherent to the polypropylene resins in 
addition to the limited improvement of the transparency of the sheet. 
For these reasons, the trend of the technology is in the direction of 
improving the transparency of the containers shaped of the resin sheet by 
thermoforming and not in the direction of improving the transparency of 
the polypropylene resin sheet per se. 
The sixth method, i.e., Japanese Patent Publication 57-17689 discloses a 
method for the fabrication of a transparent container of a polypropylene 
resin sheet by pressure forming at a temperature lower than the melting 
point thereof after heating of the resin sheet at a temperature of the 
melting point or higher followed by quenching. In this method, however, 
the resin sheet shaped in advance is reheated so that disadvantages are 
unavoidable in the degradation of the resin by the second heating and 
unevenness in heating if not to mention the costs for the increased energy 
consumption. Moreover, another problem is in the difficulty of obtaining 
uniform quenching which results in an insufficient degree of improvement 
in the transparency of the sheet and formation crimps and slackenings in 
the resin sheet in the course of the thermoforming. 
A seventh method has been proposed in which a thermoplastic resin sheet is 
imparted on both surfaces with a surface roughness of 0.7 .mu.m RMS or 
smaller and the resin sheet is unidirectionally stretched in a stretch 
ratio of 3 times or less followed by thermoforming (see, for example, 
Japanese Patent Kokai No. 53-128673). This method, however, has no 
contribution at all to the improvement of the internal haze in addition to 
the difficulty in reducing the surface roughness on both of the surfaces 
of the sheet. Therefore, the resin sheet treated by this method cannot 
have a sufficiently improved transparency. Moreover, this method is 
effective in the improvement of the transparency only when the stretch 
ratio is as high as 1.5 to 2.5 times while such a high stretch ratio is 
detrimental greatly to the thermoformability of the resin sheet. In 
addition, the transparency can be increased only by a deep drawing to some 
extent so that tearing of the sheet in the MD direction and non-uniformity 
in the transparency are unavoidable as a result of the unevenness in 
drawing. The resin sheet obtained by this method has a relatively low heat 
resistance due to the orientation therein so that the sheet is not 
suitable for fabrication by vacuum forming which necessitates a relatively 
high thermoforming temperature. 
As is understood from the description given above in detail, in the prior 
art, there are known absolutely no sheet products of a crystalline 
thermoplastic resin or, in particular, polypropylene-based resin having 
excellent transparency and surface properties along with low orientation 
and excellent thermoformability. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a novel 
method for the preparation of a resin sheet free from the above described 
problems and disadvantages in the prior art methods, according to which a 
sheet material of a crystalline thermoplastic resin having good 
transparency and high gloss comparable to those of a polyvinyl chloride 
resin sheet and excellent thermoformability can readily be obtained even 
by slightly stretching. 
Thus, the method of the present invention for the preparation of a 
crystalline thermoplastic resin sheet comprises the steps of: (a) 
extruding a melt of a crystalline thermoplastic resin into a sheet; (b) 
quenching the thus extruded molten resin sheet by introducing the same 
into a slit where cooling water is flowing; and (c) stretching the thus 
quenched resin sheet at a temperature lower than the melting point of the 
resin in a stretch ratio in the range from 1.02 to 2.00 times. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the method of the invention, various types of crystalline thermoplastic 
resins can be used as the starting base resin of the sheet including 
polyolefins such as polypropylenes, random or block copolymers of 
propylene and an .alpha.-olefin of 30% by moles or less, polyethylenes, 
random copolymers of ethylene and an .alpha.-olefin of 30% by moles or 
less, polybutenes and the like; polyesters such as poly(ethylene 
terephthalate), poly(butylene terephthalate) and the like; polyamides such 
as nylon 6, nylon 6,6, nylon 6,10 and the like; poly(phenylene sulfide); 
polyether ether ketones and others. Among them, particularly satisfactory 
results can be obtained by use of a polyolefin resin or, in particular, a 
polypropylene-based resin. The polypropylene-based resin here implied is 
exemplified, preferably, by homopolymers of propylene, random copolymers 
of propylene and an .alpha.-olefin and mixtures thereof. The 
polypropylene-based resin should have a melt index in the range from 0.2 
to 20 g/10 minutes or, preferably, from 0.3 to 15 g/10 minutes. When the 
melt index of the resin is lower than 0.2 g/10 minutes, disadvantages are 
caused by the unduly low rigidity of the resin sheet to be obtained in 
addition to the decrease in the productivity due to the decreased rate of 
extrusion out of the extruder machine. When the melt index is higher than 
20 g/10 minutes, on the other hand, difficulties are encountered in 
shaping of the sheet due to the unduly low melt viscosity. It is optional 
that the starting base resin is admixed according to need with various 
kinds of additives including nucleating agents, lubricants, antioxidants, 
ultraviolet absorbers, radiation resistance agents, antistatic agents, 
coloring agents and the like. The nucleating agent above mentioned is 
exemplified by aromatic and aliphatic carboxylic acids, dibenzylidene 
sorbitol, silica flour, talc and the like. Further, the base resin may be 
blended with a petroleum resin, adhesive resin, elastomer and the like. 
The first step of the inventive method is the melt-extrusion of the above 
described crystalline thermoplastic resin in the form of a sheet. The most 
conventional method for the melt-extrusion of a resin is the use of a 
screw extruder machine in which the resin feed is continuously melted and 
kneaded and then the resin melt is extruded through a T-die in the form of 
a sheet. As to the type of the extruder used in the inventive method, it 
is preferable to use a machine free from the problem of an excessively 
large shearing stress which is capable of kneading and extruding at a 
relatively low resin temperature and provided with a screw having a 
sufficiently large space for stress relaxation at the front end of the 
extruder machine. 
Although the extrusion is basically performed of a single kind of the 
thermoplastic resin into an extruded sheet, it is optional, if desired, to 
perform bilayer or multilayer coextrusion using a combination of different 
resins by use of two or more extruder machines. The resins to be combined 
are, for example, a homopolymerized polypropylene and a random-polymerized 
polypropylene, polypropylenes having different values of melt index, a 
polypropylene and an adhesive polyolefin modified with an unsaturated 
carboxylic acid or a derivative thereof, a polypropylene and a 
polyethylene or an ethylene-vinyl acetate copolymer, a polypropylene and 
an ethylene-vinyl alcohol copolymer, and the like. 
It is essential in the inventive method that the extrusion is performed 
under such conditions that the extruded sheet still in a molten state at 
the exit of the T-die has high transparency and smooth surfaces and is 
subject to little swelling with sufficiently relaxed stress. To say 
particularly, the extruder machine should be provided with a screw 
composed of a low-compression screw part, shearing part and 
stress-relaxation part and the extrusion is performed by keeping the resin 
at a somewhat lower temperature and after the stress has been relaxed. The 
temperature at the die exit should be controlled by use of a die-lip 
heater to be higher by 10.degree. to 60.degree. C. than the resin 
temperature. The die should be free of mars and scratches on the surface 
so as to give an extrusion-molded sheet having smooth surfaces and 
excellent transparency. 
In the next step, the thus extruded transparent resin sheet is quenched by 
introducing the same into a slit where cooling water is flowing. Namely, 
cooling water is supplied to the slit to flow therethrough and the 
extruded resin sheet still in a molten state is introduced into the slit 
along the flowing direction of the cooling water to effect quenching. In 
this case, the flowing velocity of the cooling water through the slit 
should be larger than the running velocity of the resin sheet therethrough 
or, preferably, the flowing velocity of the cooling water should be twice 
or more than that of the resin sheet. The material forming the slit is not 
particularly limitative and suitable materials include metals, plastics, 
wood, fabrics and the like provided, if necessary, with a coating layer of 
plated or vacuum-deposited metal, polytetrafluoroethylene and the like. 
The slit here implied is formed of a pair of oppositely facing surfaces 
with a narrow gap therebetween and the structure thereof is not 
particularly limitative. For example, the slit may be formed of a pair of 
oppositely facing endless belts or of a pair of rollers. Preferably, two 
stages or more of slits are provided so that the effect of quenching is 
further increased to give high-quality products with high productivity. 
Slits disclosed in the specification and the drawings of the U.S. patent 
application Ser. No. 535,661 filed September 26, 1983 now U.S. Pat. No. 
4,548,778, can be utilized as a preferable slits. The width of the slit is 
not particularly limitative but, in particular, the first-stage slit 
should have a width of 20 mm or smaller or, preferably, 10 mm or smaller 
or, more preferably, 6 mm or smaller. The height of the slit should be at 
least 3 mm or, preferably, at least 5 mm. 
In supplying the cooling water to the first-stage slit, it is preferable to 
avoid contacting of the running molten resin sheet with the surface of the 
cooling water which is stagnant or in a low flowing velocity forming a 
pool on the upper part of the slit. It is essential therefor to maintain 
the level of the cooling water in the upper part of the slit at an as low 
level as possible. Furthermore, it is important to provide a control means 
so that the first contacting line between the cooling water and the 
running molten resin sheet should be uniform and without fluctuation. 
Although the above mentioned conditions are important in the first-stage 
slit for the initial quenching in the slit water-quenching according to 
the inventive method, the influences of the conditions of the water stream 
are less significant in the second-stage and subsequent slits so that no 
particular control means may be provided for the water level and flow 
velocity. 
Water containing no particular additive may be used as the cooling water 
but it is optional that the cooling water is admixed with a surface active 
agent or an organic or inorganic thickening agent when improvements in the 
uniformity of quenching and smoothness of the surfaces of the resin sheet 
are desired. 
As organic thickening agents, various compounds such as natural polymeric 
substances, and synthetic substances (including semi-synthetic substances) 
can be used. 
Examples of natural polymeric substances include starches such as potato 
starch, sweet potato starch, wheat starch, etc.; mannans such as konnyaku; 
seaweeds such as agar, sodium alginate, etc.; viscous substances 
originated in plant such as tragacanth gum, gum arabi, etc.; viscous 
substances originated in microorganism such as dextrin, levan, etc.; and 
proteins such as glue, gelatin, casein, collagen, etc. Examples of 
semi-synthetic substances include celluloses such as viscose, methyl 
cellulose, carboxymethyl cellulose, etc.; starch substances such as 
soluble starch, carboxymethyl starch, dialdehyde starch, etc. Examples of 
synthetic substances include polyethylene glycol, polyvinyl alcohol, 
polymer of sodium acrylate, polyethylene oxide, etc. 
Examples of inorganic thickening agents are silica sol, alumina sol, clay, 
water glass, and various metal salts. 
The viscosity of an aqueous solution of an organic or inorganic thickening 
agent is from 2 to 3,000 centipoises and preferably from 3 to 1,000 
centipoises. 
In the present invention, the cooling water is used to cool the 
sheet-shaped thermoplastic resin. The temperature of the cooling water is 
suitably within the range of from -10.degree. to +50.degree. C. In the 
production of sheet having a thickness of at least 0.2 millimeter, the 
formation of haze dots can be effectively prevented by controlling the 
temperature of the cooling water to 20.degree. C. or lower and preferably 
10.degree. C. or lower. In this manner, a crystalline thermoplastic resin 
sheet is obtained by quenching the molten resin sheet to a temperature of, 
usually, 100.degree. C. or below or, preferably, 60.degree. C. or below. 
Although the above described quenching in the inventive method is 
effective in obtaining a resin sheet having excellent transparency, the 
degree of the effect obtained by quenching depends on the thickness of the 
resin sheet. For example, the haze of a resin sheet having a thickness of 
0.5 mm or smaller can be 10% or smaller or, in some cases, 5% or smaller. 
It is, however, possible to fabricate a transparent box or container by 
the subsequent thermoforming of the resin sheet regardless of the 
thickness of the sheet when the sheet has an external haze of 5% or less. 
Accordingly, it is desirable to control the conditions of extrusion and 
water quenching so that the extruded and quenched resin sheet should have 
an external haze of 5% or less. 
The next step is the stretching of the thus obtained resin sheet imparted 
with excellent surface condition and crystalline state by the well 
controlled quenching treatment at a temperature lower than the melting 
point of the crystalline thermoplastic resin in a stretch ratio in the 
range from 1.02 to 2.00 times to give a resin sheet of excellent 
transparency. In performing the stretching treatment, the resin sheet is 
heated at a temperature lower by 5.degree. to 70.degree. C. or, 
preferably, 5.degree. to 50.degree. C. than the melting point of the resin 
and subjected to stretching or rolling. This roll stretching can be 
carried out by stretching the sheet between rollers rotating at different 
speeds. Heating of the resin sheet can be performed continuously by a 
conventional method such as hot air heating, radiation heating, passing on 
a hot roll and the like. The stretching may be either uniaxial or biaxial 
although quite satisfactory results can be obtained usually by uniaxial 
stretching. The stretch ratio should be in the range from 1.02 to 2.00 
times or, preferably, in the range from 1.03 to 1.50 times although the 
optimum stretch ratio depends on the temperature and manner of stretching 
and should be determined according to the intended application, thickness 
of the sheet and other factors. Stretching of the sheet can be performed 
in several different ways with no particular limitation. For example, 
rolling is preferred when the resin sheet as such is used as a base 
material of paper folders, bent-sheet boxes and the like products since 
the method is effective in eliminating the fine die lines and other 
defects on the surface of the sheet. The rolling of a resin sheet is 
performed by passing the sheet through a narrow gap smaller than the 
thickness of the sheet between a pair of rollers each having a smooth 
surface with a low surface roughness and rotating in a reverse direction 
to the other. When the resin sheet is to be used in the fabrication of a 
container by thermoforming at a temperature below the melting point of the 
resin, the preferable stretch ratio is in the range from 1.02 to 2.00 
times for pressure molding and in the range from 1.02 to 1.10 times for 
vacuum forming. A shrinking stress of 0.2 to 30 kg/cm.sup.2 is usually 
sufficient by this stretching of the resin sheet though dependent on the 
conditions of stretching. Namely, stretching of only a very low degree is 
sufficient in the inventive method because the purpose of the stretching 
is not only to improve the transparency of the resin sheet but to 
eliminate curlings and crimps in the quenched sheet and to prevent 
formation of slackening and crimps in the course of the thermoforming. 
It is optional in the method of the invention that the above described step 
of stretching is preceded by a heat treatment of the quenched resin sheet. 
Namely, a step of heat treatment is added between the quenching step, in 
which the molten resin sheet is quenched by being introduced into a slit 
where cooling water is flowing, and the step of stretching. 
The heat treatment here implied is performed by heating the resin sheet 
using a hot roller, hot air, hot inert liquid and the like at a 
temperature lower than the melting point of the base resin by 10.degree. 
to 60.degree. C. or, preferably, by 20.degree. to 50.degree. C. In other 
words, this heat treatment has an effect of annealing and contributes to 
the further improvement of the transparency and rigidity of the resin 
sheet. 
According to the method described above in detail, a crystalline 
thermoplastic resin sheet can be obtained which is excellent in the 
transparency, surface properties and appearance and has good formability 
suitable for the fabrication of containers by the techniques of vacuum 
and/or pressure forming. The thickness of the resin sheets which can be 
manufactured by the inventive method is not limited to a particular range 
but may include a thickness in the range from 0.05 to 2 mm or, preferably, 
from 0.1 to 1 mm. When the thickness of the sheet is 0.5 mm or smaller, 
the lowest value of the haze is 20% or below or, in some cases, 10% or 
below. 
The crystalline thermoplastic resin sheet prepared by the inventive method 
has transparency and appearance quite different from those of the 
conventional crystalline thermoplastic resin sheets and, when the 
thermoplastic resin is a polypropylene resin, is not distinguishable by 
the appearance alone from the sheets of polyvinyl chloride resins and 
polystyrene-based resins. In addition to the indistinguishableness of 
appearance from the conventional amorphous thermoplastic resin sheets, the 
resin sheet prepared by the inventive method is excellent in strength or, 
in particular, strength against bending and folding and in the absence of 
the phenomenon of whitening so that the application of the resin sheet of 
the invention is not limited to paper folders and other stationery 
products, which have been conventionally fabricated of the amorphous 
thermoplastic resin sheets, but includes various containers fabricated by 
bending and folding of a resin sheet and many other fields as a 
possibility of future development. 
The crystalline thermoplastic resin sheet prepared by the inventive method 
can be quite satisfactorily used in the fabrication of various kinds of 
molded articles such as containers by a known technique of thermoforming 
such as vacuum forming, pressure forming, plug assist pressure forming, 
matched-mold thermoforming and the like at a temperature below the melting 
point of the resin or, usually, at a temperature lower than the melting 
point by 5.degree. to 50.degree. C. The thermoforming can be performed at 
a relatively high temperature by virtue of the unique characteristic 
features of the resin sheet prepared by the inventive method including the 
high transparency already imparted to the sheet, the fine crystallite size 
in the resin sheet controlled by means of the quenching and the very low 
degree of stretching, i.e. very low degree of orientation, in the sheet 
which means the good thermoformability of the resin sheet. The shape 
reproducibility in the thermoforming is excellent even under a relatively 
low forming pressure as in the vacuum forming which is quite 
satisfactorily applicable to the resin sheet of the invention. Therefore, 
relatively simple and inexpensive forming machines and molds can be used 
in the thermoforming of the resin sheet by the inventive method. In 
comparison with conventional crystalline thermoplastic resin sheets in 
which transparency can be exhibited only by the stretching, and 
orientation in the course of the thermoforming, the resin sheet prepared 
by the inventive method is highly transparent a such so that no stretching 
and orientation of the resin sheet may be involved in the thermoforming 
and highly uniform containers without unevenness in the transparency can 
be fabricated of the resin sheet including shallow containers and 
deep-drawn containers as well as containers of complicated forms. 
Furthermore, the low degree of orientation in the thus fabricated 
containers gives an advantage of a high temperature of the incipient heat 
shrinkage of the container in use so that development of the application 
thereof to new fields can be expected such as the packaging container of a 
retortpouched food and the container for melt-filling packaging of a 
heat-flowable food such as candies. Needless to say, the above mentioned 
applications are only several of special uses and the resin sheet prepared 
by the inventive method can be used quite satisfactorily in the 
fabrication of containers for foods and beverages in general, containers 
for press-through-package of medicines and confections, containers for 
blister package and the like. 
In the following, the method of the present invention is described in more 
detail by way of examples.

EXAMPLES 1 to 7 
An isotactic polypropylene having a melting point of 168.degree. C. and a 
melt index of 2.0 g/10 minutes was melted and extruded into a transparent 
molten resin sheet at a resin temperature of 250.degree. C. out of a T-die 
of 550 mm width and a die lip opening of 1 mm provided with a die lip 
heater and mounted on an extruder machine having a diameter of 65 mm and 
L/D=28. The extruded molten resin sheet was introduced into a water 
quenching unit A or B described below in which the molten resin sheet was 
quenched into a polypropylene resin sheet under the conditions shown in 
Table 1. The resin sheet was then heated to 130.degree. C. on a heating 
roller and stretched in a stretch ratio indicated in Table 1 to give a 
stretched polypropylene resin sheet. The tensile modulas of the thus 
stretched resin sheet was 19000 kg/cm.sup.2. 
The stretched resin sheet was subjected to thermoforming in the conditions 
indicated in Table 1 to evaluate the formability of the resin sheet and 
the quality of the thermoformed containers to give the results shown in 
Table 1. 
Water quenching unit A (single-stage slit) 
The slit had a height of 50 mm and a width of 3 mm and the water level of 
the water pool above the slit was 5 mm with cooling water at a temperature 
of 2.degree. C. 
Water quenching unit B (two-stage slits) 
The first-stage slit had a height of 50 mm and a width of 2 mm and the 
water level of the water pool above the slit was 5 mm. The second-stage 
slit had a height of 10 mm and a width pf 4 mm and the water level of the 
water pool above the slit was 10 mm with cooling water at a temperature of 
10.degree. C. 
TABLE 1 
__________________________________________________________________________ 
Example Comparative 
1 2 3 4 5 6 7 Example 1 
__________________________________________________________________________ 
Sheet Quenching unit 
B B B B B B A B 
molding 
Rate of extru- 
20 20 20 20 20 20 10 20 
sion, m/minute 
Thickness, mm 
0.45 
0.36 
0.35 
0.33 
0.32 
0.31 
0.35 
0.30 
Haze, % 19 17 17 16 15 15 25 15 
Stretching 
Stretch ratio 
1.50 
1.20 
1.15 
1.10 
1.05 
1.03 
1.15 
-- 
Haze, % 9 8 7 7 6 6 13 -- 
Shrinking 
-- -- 15.0 
-- -- 0.3 
-- -- 
stress, kg/cm.sup.2 
Forming 
Type of *.sup.1 
*.sup.1 
*.sup.2 
*.sup.2 
*.sup.1 
*.sup.1 
*.sup.2 
*.sup.2 
of thermoforming 
container 
Thickness of 
0.17 
0.18 
0.15 
0.16 
0.17 
0.16 
0.16 
0.17 
container wall, 
mm 
Haze, % 2 2 3 2 3 2 7 5*.sup.3 
__________________________________________________________________________ 
*.sup.1 The container was shaped by pressure forming using a metal mold 
having 10 cavities each in a semiellipsoidal form having a depth of 15 mm 
and an opening of 25 mm by 15 mm under conditions of a hot plate 
temperature of 140.degree. C. and a forming pressure of 4 kg/cm.sup.2. 
*2: *.sup.2 The container was shaped by vacuum forming using a metal mold 
having 10 cavities each in a semiellipsoidal form having a depth of 10 mm 
and an opening of 25 mm by 15 mm at a sheet temperature of 157.degree. C. 
*.sup.3 Crimps were formed on the container to impart a poor appearance 
thereto. 
**Stretch ratio (sheet) Thermoformability 
In example 2 instead of the pressure forming, vacuum forming was 
undertaken at a forming temperature of 155.degree. C. without success. 
Containers could be shaped by increasing the molding temperature to 
165.degree. C. although the container were very poor in the transparency 
and surface gross. 
EXAMPLE 8 
The experimental procedure was substantially the same as in Example 1 
except that the isotactic polypropylene was replaced with a high-density 
polyethylene having a density of 0.960 g/cm.sup.3, melting point of 
130.degree. C. and melt index of 0.2 g/10 minutes and the temperature of 
the resin sheet under stretching was 110.degree. C. to give a polyethylene 
sheet having excellent transparency. 
COMATIVE EXAMPLE 1 
The experimental procedure was substantially the same as in Example 3 
except that the step of stretching was omitted. Table 1 also gives the 
results obtained in the evaluation of the formability of the resin sheet 
and the quality of the thermoforming container. 
EXAMPLE 9 
The water-quenched resin sheet in Example 1 was preheated at 140.degree. C. 
and then subjected to rolling in a rolling ratio of 1.2 by passing through 
a pair of pressure rollers heated at 110.degree. C. The thus obtained 
rolled sheet was free from curling, crimps, slackening, die lines and 
microfisheyes in comparison with the same sheet before rolling and 
excellent in apperance and transparency with a haze value of 7%. 
EXAMPLES 10 TO 16 
A homopolymeric polypropylene resin having a density of 0.91 g/cm.sup.3, 
melting point of 165.degree. C. and melt index of 2.1 g/10 minutes was 
melted and extruded at a resin temperature of 240.degree. C. into a 
transparent molten resin sheet out of a T-die of 730 mm width and a die 
lip opening of 1.5 to 3 mm provided with a die lip heater and mounted on 
an extruder machine having a diameter of 90 mm with L/D=28. The thus 
extruded molten resin sheet was quenched by continuously introducing into 
a water quenching unit equipped with two-stage slits, of which the 
first-stage slit had a height of 50 mm and a width of 2.5 mm and the water 
level of the water pool above the first-stage slit was 5 mm with cooling 
water at 5.degree. C. while the second-stage slit had a height of 10 mm 
and a width of 5 mm and the water level of the water pool above the 
second-stage slit was 10 mm with cooling water at 5.degree. C. The 
conditions of extrusion were varied so that the thus quenched 
polypropylene sheets were varied in thickness. Each of the thus obtained 
polypropylene sheets was subjected to a heat treatment by passing through 
a heat-treatment unit composed of 4 rollers each having a diameter of 300 
mm and heated at 145.degree. C. followed by rolling in a roll ratio of 1.1 
by passing through a pair of pressure rollers each having a diameter of 
200 mm and heated at 120.degree. C. to give polypropylene sheets for 
thermoforming having different values of thickness as shown in Table 2. 
These sheets were subjected to the measurements of the haze value and 
physical properties to give the results shown in Table 2. No curling and 
crimps were found in these sheets. 
With an object to evaluate the thermoformability of these resin sheets and 
the transparency of the containers thermoformed thereof, each of the 
sheets was fabricated into approximately cylindrical containers having a 
diameter of 50 mm and a varied depth by several different molding methods 
indicated in Table 2 which summarizes the results obtained in the 
evaluation of the formability of the resin sheets and the measurements of 
the properties of the containers. 
COMATIVE EXAMPLE 2 
The experimental procedure was substantially the same as in Example 12 
except that the heat treatment and the rolling treatment were omitted. The 
results of the evaluation undertaken in the same manner as in Example 12 
are shown in Table 2. 
TABLE 2 
__________________________________________________________________________ 
Example Comparative 
10*.sup.1 
11 12 13 14 15 16 Example 2 
__________________________________________________________________________ 
Sheet Thickness, mm 
0.4 0.4 0.55 
0.55 
0.60 
0.80 
1.00 
0.55 
Haze, Total 5.7 5.7 16.3 
16.3 
22.3 
54.8 
74.8 
34.2 
% Internal 
4.7 4.7 15.9 
15.9 
21.0 
54.1 
73.2 
33.6 
External 
1.0 1.0 0.4 0.4 1.3 0.7 1.6 0.6 
Properties 
Tensile strength, 
555/ 
555/ 
548/ 
548/ 
496/ 
489/ 
493/ 
430/ 
of sheet 
kg/cm.sup.2 
470 470 473 473 473 477 475 438 
(MD/TD) 
Ultimate elonga- 
400/ 
400/ 
410/ 
410/ 
430/ 
430/ 
430/ 
425/ 
tion, % 460 460 440 440 430 450 430 480 
Tensile modulus, 
19000/ 
19000/ 
19000/ 
19000/ 
19000/ 
19000/ 
19000/ 
10800/ 
kg/cm.sup.2 
18000 
18000 
19000 
19000 
19000 
18000 
19000 
10600 
Conditions 
Shaping method 
*.sup.2 
*.sup.3 
*.sup.2 
*.sup.3 
*.sup.3 
*.sup.3 
*.sup.3 
*.sup.2 
of thermo- 
Depth of shaped 
13 13 25 25 50 100 100 25 
forming & 
body, mm 
results 
Formability 
*.sup.4 
Good 
*.sup.4 
Good 
Good 
Good 
Good 
*.sup.4 
Properties 
Side 
Thickness, 
0.25 
0.22 
0.23 
0.16 
0.13 
0.15 
0.19 
0.25 
of wall 
mm 
container Haze, % 
2.3 2.8 5.3 6.5 8.2 8.3 9.1 10.1 
Bot- 
Thickness, 
0.19 
0.20 
0.16 
0.19 
0.16 
0.21 
0.25 
0.14 
tom mm 
Haze, % 
1.8 2.3 1.9 2.1 2.0 2.5 3.6 5.8 
__________________________________________________________________________ 
*.sup.1 Extrusion velocity in sheet preparation: 16 meters/minute; averag 
flow velocity of cooling water in slit: 59 meters/minute. 
*.sup.2 Vacuum forming with the sheet heated at 153 to 158.degree. C. 
*.sup.3 Plug pressure forming with the sheet heated at 135 to 140.degree. 
C. under a pressure of 3 kg/cm.sup.2. 
*.sup.4 Somewhat smaller thickness at bottom.