Heat-insulating corrugated cardboards and method for making them

A corrugated cardboard includes a corrugated core sheet comprising a plastic sheeting having thereon a skin layer having a specific cell diameter and a plastic film laminated on at least one side of the corrugated sheet. This corrugated cardboard is improved in terms of heat insulation and so can be used for packing such materials as petals and perishables, while protecting them against heat and keeping them fresh. More preferably, shielders may be provided in the corrugated core sheet grooves. A specific method for making such cardboard products is also provided.

BACKGROUND OF THE INVENTION 
The present invention relates to a packaging corrugated cardboard which is 
both lightweight and excellent in heat insulation. Specifically, this 
invention concerns a heat-insulating corrugated cardboard of great 
water-resistant strength, characterized in that its corrugated core is 
formed of a foamed plastic sheet which may be laminated on one side or 
both sides with a plastic film or films, and a method for making such a 
cardboard. More specifically, this invention is directed to a 
heat-insulating corrugated cardboard designed to be used as the packaging 
material which is required to protect petals or perishables against heat 
while keeping them fresh and/or to have resistance to scratching and 
scuffing and shock absorption, and a method for making it. 
PRIOR ART 
So far, box members formed of foamed polystyrene resin by injection molding 
have been used in fields for which high degrees of heat insulation are 
demanded. These members excel in heat insulation, but need to keep an 
array of separate frames for different box sizes and shapes in store. Such 
frames, because of their high assembly cost, have made batch production 
systems other than mass production systems impossible. Due to their 
considerable thickness and inability to be folded up, the foamed 
polystyrene resin-made box members have had an additional defect of 
needing much space for transportation and storage. Still other defects 
have been that when a plurality of packed boxes are stacked up, the 
underlying ones are likely to break down, deform or collapse due to a 
strength shortage. 
Conventional corrugated cardboards, on the other hand, have had the 
advantage of being able to be folded up and carried in sheet forms to 
places where they are packed and occupy less space during storage, but 
their disadvantage has been that they are far from satisfactory in terms 
of heat insulation. 
In recent years, corrugated cardboards in which either one of both their 
liners is laminated on its outer or inner face with a foamed plastic sheet 
have been proposed. However, their heat insulation is still less than 
satisfactory. 
Nor is there any package to protect perishables or petals against heat 
while keeping them fresh. 
In view of the defects associated with the prior art, it is an object of 
this invention to provide a heat-insulating corrugated cardboard which is 
small in volume and light in weight, accommodates easily to size and shape 
variations, is folded up into a sheet which is easily placed on another 
during transportation and storage, is readily assembled into the required 
shape in use and is much superior in heat insulation to conventional ones, 
or a corrugated cardboard which is much more effective for protecting 
flowering plants or perishables against heat while keeping them fresh, as 
compared with conventional ones, or a corrugated cardboard which protects 
the content against scratching/scuffing and shock, possesses high 
water-resistant strength and a suitable degree of moisture permeability 
and has an internal humidity regulating function. Another object of this 
invention is to provide a method for making such corrugated cardboards. 
SUMMARY OF THE INVENTION 
According to one aspect of this invention, the above-mentioned object is 
achieved by the provision of a corrugated cardboard in which a foamed 
plastic sheet provided with a specific skin layer and having a closed-cell 
content of at least 70% is used as the corrugated core sheet. 
Another aspect of this invention is directed to a corrugated cardboard 
according to the first aspect, wherein a plastic film or films is or are 
laminated on one side or both sides of the foamed plastic sheet, thereby 
providing a package member which is light in weight, excels in heat 
insulation, has more improved water-resistant strength and insures 
improved protection against heat and freshness. 
A third aspect of this invention is directed to a corrugated cardboard in 
which shielders are provided in grooves in the corrugated core sheet to 
prevent vagrant movement of air in it, thereby achieving much more 
improved protection against heat. In this connection, it is understood 
that the use of a skin layer-free foamed plastic sheet for the corrugated 
core sheet is somewhat, if not seriously, less effective. 
A forth aspect of this invention is directed to a corrugated cardboard 
according to any one of the first to third aspects, which is laminated on 
one side with a foamed plastic sheet, thereby providing more improved 
protection of the matter packed against heat, scratching/scuffing and 
shock. 
A fifth aspect of this invention is directed to a method for making a 
corrugated cardboard according to any one of the first to fourth aspects. 
Taken altogether, the present invention contemplates improving the heat 
insulation of a corrugated cardboard, cutting off its air permeation and 
increasing its compression strength by laminating a plastic film or films 
on one side or both sides of its corrugated core sheet.

DETAILED DESCRIPTION OF THE INVENTION 
The heat-insulting corrugated cardboard according to this invention is 
characterized in that its corrugated core material 2 is in a sheet form 
having a thickness of 0.7 to 7.0 mm. At less than 0.7 mm any 
heat-insulting effect is not expected, while at higher than 7.0 mm 
sufficient heat insulation is achieved, but difficulty is involved in 
corrugation processing. In this invention, it is essentially required that 
the skin layer is in 4 to 150 .mu.m in average cell diameter, as measured 
in section, and applied at least on one side of the core sheet 2 at an 
average thickness of 100 .mu.m to 700 .mu.m inclusive, and the core sheet 
be 10 to 1,000 .mu.m, preferably 10 to 500 .mu.m in average cell diameter, 
as measured in section. Larger cell diameters are not desirable due to a 
drop in heat insulation. 
By definition, what tells the skin layer from the core sheet is at their 
average cell diameter ratio of 1:1.8. More specifically, the core sheet 
material comprises a foamed plastic sheet 2 which is at least about 1.8 
times as great as the skin layer in terms of average cell diameter, as 
measured in section, and has an average foaming factor of 8 to 40. The 
plastic sheet 2 is bonded on both sides with paperboard liners 1. In 
addition, the corrugated core sheet 2 may be provided with shielders 3 in 
its grooves, and may further be laminated on one side with a foamed 
plastic sheet 4 through the paperboard liner 1 (see FIG. 6). It is noted 
that the mean cell diameter in section of the skin layer and of core 
sheeting of the corrugated core sheet 2 are defined to be a 
divident/divisor, the divident being the total size in section of cells in 
the skin layer and core sheeting of the corrugated core sheet 2 and the 
divisor being the number of cells. In the present invention, the mean 
foamed factor of the foamed plastic sheet is fixed at 8 to 40 for the 
reasons that only poor heat-insulating effects are achieved at a factor of 
less than 8, while there is an increase in the cell diameters and hence a 
lowering of heat insulation at a factor of higher than 40. 
FIGS. 7 and 8 shows two embodiments of this invention wherein a plastic 
film or films 5 is or are laminated on one or both sides of a corrugated 
core sheet 2. The plastic film 5, for instance, may be formed of 
polyester, polyethylene, polystyrene, polypropylene, polyamide, nylon, 
polyvinyl alcohol, vinyl chloride, vinylidene chloride and all other 
available plastics. The lamination of the plastic film serves well to 
prevent its collapse owing to a pressure used for bonding the associated 
liners to the corrugated core sheet. 
Preferably, the plastic film should have a thickness of 5 to 50 .mu.m. At 
below 5 .mu.m, it is likely to crimp or break, thus giving rise to not 
only a processing problem but also a limited improvement in compression 
strength. At higher than 50 .mu.m, on the other hand, it contributes to an 
increase in compression strength, but results in cost rises. Thus, the 
upper thickness limit is fixed at 50 .mu.m from the economical standpoint. 
As described above, the corrugated core sheet 2 of the heat-insulating 
corrugated cardboard according to this invention includes at least one 
side a skin layer having a mean cell diameter of about 4 to 150 .mu.m at a 
thickness of about 100 to 700 .mu.m, with the core sheeting being formed 
of a foamed plastic sheeting by corrugation processing, having a mean cell 
diameter of 10 to 1,000 .mu.m in section and being at least about 1.8 
times as high as the mean cell diameter in section of the skin layer, and 
has paperboard liners 1 bonded to its both sides. To make clear the effect 
of the skin layer provided on the corrugated core sheet 2, two 
uncorrugated, 3 mm thick flat sheets of foamed polyethylene, one having a 
skin layer and the other otherwise, were tested for their insulation 
effectiveness according to ASTM method. From the results, reported below, 
it has been found that the skinned sheet is much superior in insulation 
effectiveness to the control sheet. 
______________________________________ 
Foamed Polyethylene 
(A) Skinned (B) Control 
______________________________________ 
1. Mean cell diameter 
81 -- 
of skin layer in .mu.m 
2. Skin layer thickness 
100 to 150 -- 
in .mu.m 
3. Mean cell diameter 
258 304 
of core sheeting in .mu.m 
4. Insulation effec- 
54.7 50.9 
tiveness in % 
5. Mean foaming 
about 30 about 30 
factor (26.7 measurements) 
(27.5 measurements) 
6. Thickness in mm 
about 3 about 30 
(2.9 measurements) 
(2.8 measurements) 
______________________________________ 
Then, the effect of the shielders 3 on the insulation effectiveness of a 
box package was measured, which included a corrugated core sheet 2 
obtained by corrugating a flat foamed plastic sheet. 
Referring it in more detail, a 20 cm long corrugated cardboard under test 
was formed, with a synthetic rubber-based banding agent, into a hexahedron 
with each plane of 20 cm.times.20 cm in size (one plane of which was 
blanked out)--see FIG. 9, and cellophane tapes were applied to the apexes 
to prevent any possible air leakage through them. There was provided an 
about 10 cm thick block 6 (FIG. 10) of foamed polystyrene, which was such 
grooved--at 7--to a depth of about 1 cm as to receive the above-mentioned 
hexahedral box. Then, a bulb socket was mounted at the central region of 
the block 6 such that the filament of an incandescent electric lamp, once 
in place, was located about 7 cm above the upper plane of the block 6 (see 
FIG. 10). Finally, thermocouples for measuring temperature (provided with 
a light shielding cover for eliminating the influence of direct radiant 
heat from the lamp) were placed near the the upper corners of the inner 
sides of the hexahedral box. 
The hexahedrall corrugated cardboard box under test was set in the groove 7 
in the block 6 with the blanked-out plane down. Then, the in-box 
temperature was measured at time intervals of 2 seconds by the 
thermocouples while the incandescent lamp was powered at a power 
consumption regulated to 25 W with a d.c. stabilization power source. 
After the constant temperature had been reached, an additional long 
heating was continued at the regulated power consumption to obtain 
temperature readings. It is noted that the higher the in-box temperature 
readings, the better the insulation effectiveness, and vice versa. 
With the above-described testing equipment and procedure, measurement was 
made of corrugated core sheets which were formed of normal paperboard and 
provided with shielders at varied intervals. The results are set out 
below. 
______________________________________ 
Spaces between Without 
shielders (cm) 
1 3 7 10 15 shielders 
______________________________________ 
In-box tempera- 
60.4 60.7 59.5 58.7 58.4 57.3 
ture (.degree.C.) 
______________________________________ 
From the above-mentioned results, it has been found that the provision of 
the shielders in the grooves in the corrugated core sheets contributes to 
an increase in insulation effectiveness. Referring to the space between 
the shielders, it has turned out that the smaller the space, the slightly 
better the heat insulation, although this is true of a space of up to 3 
cm. At too small a space there is a lowering in heat insulation due to the 
heat conductivity of the shielders themselves. From cost-effectiveness 
analysis, however, the best results would be practically obtained by 
cutting off vagrant movement of air through the grooves in the core sheet 
at both ends of the box package. 
The foregoing are the results obtained with the corrugated core sheets 
provided on one side with shielders in the hexahedron testing manner. Set 
out below are the results obtained for the insulation effectiveness of 
corrugated core sheets provided on both sides with shielders. 
______________________________________ 
Space between shielders (cm) 
1 3 15 
______________________________________ 
One side (.degree.C.) 
60.4 60.7 58.4 
Both sides (.degree.C.) 
61.3 61.8 58.9 
______________________________________ 
As can be understood from the above-described results, it is desired that a 
corrugated core sheet be provided at both sides with shielders, because 
more insulation effectiveness was achieved with the core sheet shielded on 
both sides than with the core sheet shielded on one side, but even a core 
sheet shielded on one side would be expected to produce considerable 
insulating effects. Where some insulation effectiveness is needed with as 
thin a core sheet as possible, however, it is more advantageous to provide 
shielders on both sides of corrugated core sheets. The shielders may be 
formed on the corrugated core sheets simultaneously with, or subsequent 
to, the preparation of the latter. 
The heat insulation of various corrugated cardboards was measured according 
to ASTM method, and is set out below in terms of insulation effectiveness. 
(1) Conventional corrugated cardboard of 5 mm in thickness which had 
paperboard liners on both sides and included a corrugated core paperboard 
sheet. Insulation effectiveness: 42.7%. 
(2) Skin layer-free, corrugated core sheet of foamed polyethylene which had 
paperboard liners on both sides. Insulation effectiveness: 61.7%. 
(3) Skinned, corrugated core sheet of foamed polyethylene which had 
paperboard liners on both sides. Insulation effectiveness: 63.1%. 
From the above-mentioned results, it has been found that the provision of 
the skin layer contributes to an increase in insulation effectiveness. 
Samples (2) and (3) are both higher than Sample (1) in insulation 
effectiveness. However, the skin layer-free sample (2) is slightly lower 
than the skinned sample (3). The reason is presumably that some skin layer 
has been formed on the surface of the foamed polyethylene sheet during its 
corrugation. 
Measurement was taken of corrugated core sheets prepared by using foamed 
polystyrene in place of foamed polyethylene. 
(4) Inventive Product A 
Including a skinned, corrugated core sheet of foamed polystyrene and having 
paperboard liners on both sides. 
Insulation effectiveness: 64.0%. 
(5) Inventive Product B 
Including a skinned, corrugated core sheet of foamed polystyrene, laminated 
on both sides with films, and having paperboard liners on both sides. 
Insulation effectiveness: 64.5%. 
(6) inventive Product C 
Including a skinned and shielded, corrugated core sheet of foamed 
polystyrene and having paperboard liners on both sides. 
Insulation effectiveness: 65.0%. 
(7) Inventive Product D 
Including a skinned, corrugated core sheet of foamed polystyrene and having 
a paperboard liner on one side and a foamed polystyrene sheet laminated on 
the other side through a paperboard liner. 
Insulation effectiveness: 73.4%. 
As can be clearly seen from the above-mentioned results, the inventive 
products are much higher than the conventional corrugated cardboards in 
terms of heat insulation. 
Provided were an about 420.times.280.times.200 mm box formed of polystyrene 
by injection molding and a box made with one of the inventive products 
(which was 7 mm in thickness, had 25 .mu.m thick, high-impact films 
laminated on both sides of the core sheet and included paperboard liners 
on both sides), each containing 2 kg of a coolant. The boxes were kept 
stationary in a constant-temperature room of 20.degree. C. for 12 hours, 
followed by measuring the in-box temperatures. They were both found to be 
maintained at about 13.degree. C. or less, indicating that the inventive 
product was virtually equivalent to the injected polystyrene box in terms 
of heat insulation. 
Usually, corrugated cardboards are so largely affected by humidity that 
their strength drops. By contrast, the inventive product is so unlikely to 
be affected by humidity that its drop of compression strength by moisture 
can be largely limited. Samples pretreated according to JIS Z 0203 were 
subjected to corrugated cardboard compression strength tests according to 
JIS Z 0401 (vertical compression tests). The results are reported below. 
______________________________________ 
Compression Strength (kgf/50 mm) 
Temperature: Temperature: 
29.degree. C., Relative 
30.degree. C., Relative 
humidity: 65% 
humidity: 90% 
______________________________________ 
General currugated 
25.06 11.36 
cardboard A 
Inventive product 
53.0 39.2 
(of 5 mm in thickness and 
having a 23/100-mm core 
sheet) 
Cardboard (of 5 mm in 
60.0 43.9 
thickness and having a 
2 mm core sheet laminated 
on both sides with films) 
______________________________________ 
Note: 
Core mean cell diameter 221 .mu.m. 
Skin mean cell diameter 98 .mu.m. 
Skin thickness 110 to 250 .mu.m. 
After water dipping treatments following residual vertical strength tests 
according to JIS Z 1537, the samples were measured for their 
water-resistant strength according to the JIS Z 0401 residual vertical 
compression strength tests. The results are given below. 
______________________________________ 
Water-Resistant Strength (kgf/50 mm) 
Temperature: 20.degree. C. 
Relative 1-hr water 
humidity: 65% dipping 
______________________________________ 
General currugated 
25.06 0 
cardboard A 1 
Inventive product 
53.0 12.5 
(of 5 mm in thickness 
and having a 23/100 mm 
core sheet) 
Cardboard with core 
60.0 17.2 
sheet laminated on both 
with films sides 
______________________________________ 
Note: 
Core mean cell diameter 226 .mu.m. 
Skin mean cell diameter 102 .mu.m. 
Skin thickness 110 to 260 .mu.m. 
As can be seen from the above-mentioned results, the inventive product has 
an additional feature of being unlikely to degrade by humidity. 
With box packages obtained according to this invention, such frozen aquatic 
products as frozen lobsters or bluefins were packed for transportation and 
storage. They were found to undergo little, if any, degradation by 
hygroscopicity. Even when the boxes were stacked up over an extended 
period of time, the underlying ones did not break at all. Their improved 
heat insulation also served well to keep such products fresh. 
A package having a core sheet laminated on both sides with films is 
comparable to a JIS Grade No. 4 corrugated cardboard, and has an increased 
water-resistant strength. 
The corrugated cardboards according to this invention is particularly 
effective for keeping perishables or flowering plants fresh. 
For instance, such agricultural and aquatic products as opened and dried 
saurels were frozen for low-temperature distribution. They were placed in 
a box package according to this invention and a conventional corrugated 
cardboard package, re-frozen in an air-blast freezer at -50.degree. C. and 
subsequently allowed to stand at room temperature to monitor temperature 
rises. After the room temperature (28.degree. C.) had been reached, 
measurement was then taken of the POV value in meg/kg (a lipid or peroxide 
value determined after extraction with ethyl ether) and K value in % (a 
value determined with a K value meter after extraction with a 10% TCA 
solution and neutralization with a KOH solution or an index to how fresh 
fish and shellfish are; the lower this value, the more pleasant the taste) 
of the products. The results are given below. 
______________________________________ 
Functional POV K value 
judgement (meg/kg) (%) 
______________________________________ 
Conventional package 
yellowing found 
4.1 62.5 
Inventive package 
no yellowing 
2.9 47.4 
O: Organoleptic assay. 
______________________________________ 
Tests were also made of how long the freshness of vegetables or cut flowers 
was kept. For the purpose of delivering perennial babies'-breathes, 
provided were a general corrugated cardboard package, a package formed of 
polystyrene by injection molding and a corrugated cardboard package having 
a corrugated core sheet of foamed plastic. In terms of heat-insulating 
effects, the polystyrene and inventive packages were found to be much 
superior to the general corrugated cardboard one. Another measurement was 
taken of the concentration of ethylene gas emissions found within the 
package samples at the end of delivery, which is considered to have a 
serious influence on wilting of fresh flowering plants, etc. The 
polystyrene and usual packages had concentrations of 69 ppb and 47 ppb, 
respectively, in sharp contrast to 35 ppb found in the inventive sample; 
this indicates that the inventive sample is far superior to the control 
samples. This is because the polystyrene sample provided rapid emission of 
ethylene gas due to in-box temperature rises, while the usual sample gave 
off more ethylene gas due to its inferior heat insulation in spite of its 
good air permeation. 
A further investigation was made of how long the freshness of boiled green 
soybeans was kept during transportation. As a result, it was found that 
the inventive sample was inferior to the polystyrene sample in terms of 
heat insulation, but there was no appreciable difference between them in 
terms of the total content of sugar determined after delivery. 
Flowers were arranged in vases formed of the heat-insulating corrugated 
cardboard according to this invention and an ordinary corrugated 
cardboard. The flowers set in the vase according to this invention was 
less in the rate of reduction of moisture and so remained lush. 
______________________________________ 
Control Sample Inventive Sample 
______________________________________ 
Water reduction rate 
8.7% 2.9 to 3.7% 
Wilting of the leaves 
Wilted and headed 
not found 
downward 
______________________________________ 
The inventive sample, because of having a moderate degree of moisture 
permeation or hygroscopicity (JIS Z 0208), is unlikely to have an adverse 
influence upon the content due to moisture accumulation, unlike the foamed 
polystyrene sample. Nor is it likely that the content may be deprived of 
moisture to an overdry state. 
______________________________________ 
Moisture Permeability 
______________________________________ 
Ordinary corrugated cardboard: 
1056 g/m.sup.2 .multidot. 24 hr 
(40.degree. C. and 
90 R.H.). 
Injected, foamed polystyrene: 
55.5 g/m.sup.2 .multidot. 24 hr. 
Inventive product having no film: 
202 g/m.sup.2 .multidot. 24 hr. 
Inventive product having a core sheet 
122 g/m.sup.2 .multidot. 24 hr. 
laminated on both sides with 25-.mu.m 
thickness, high-impact polystyrene films: 
______________________________________ 
As set out above, the inventive product is designed to transmit moisture 
moderately, as expressed in terms of a moisture permeability of 122 
g/m.sup.2 .multidot.24 hr. However, even if a flat sheet of foamed 
polystyrene is provided on both sides with flat, high-impact polystyrene 
films, it is then found to transmit little or no moisture, as expressed in 
terms of a moisture permeability of 32.1 g/m.sup.2 .multidot.24 hr. 
In the description that follows, reference will now be made to how to 
prepare the heat-insulating corrugated cardboard according to this 
invention. According to the first method, use is made of a plastic sheet 
which has been subjected to primary foaming or otherwise foamed 
incompletely. This sheet is heated in foaming equipment to a deformation 
temperature plus (10 to 50) .degree.C. at which it is subjected to 
secondary foaming simultaneously with forming a skin layer on at least one 
side thereof. After that, the sheet is passed between a pair of toothed 
corrugating rolls maintained at 30.degree. to 90.degree. C. and adapted to 
be in mesh with each other, followed by cooling. The thus corrugated, 
foamed plastic sheet was applied on its apexes with a bonding agent, and 
is then passed between two rolls with paperboard liners placed on both 
sides, followed by heating. If required, the resulting sheet may be cut 
into a given size. 
Alternatively, a plastic film which has been subjected to primary foaming 
or otherwise foamed incompletely is passed down through vertically 
arranged heater equipment to heat it to a deformation temperature plus (10 
to 50) .degree.C., at which it is subjected to secondary foaming 
simultaneously with forming a skin layer on at least one side thereof. The 
thus skinned, foamed plastic sheet is then passed between a pair of 
toothed corrugating rolls, which are maintained at 30.degree. to 
90.degree. C. and located oppositely with a given space therebetween such 
that they are adapted to be in mesh with each other. Through the 
corrugating rolls the sheet makes its way horizontal, followed by cooling. 
After that, the thus corrugated, foamed plastic sheet was applied on its 
apexes with a bonding agent, and then passed between two rolls with 
paperboard liners placed on its both sides, followed by heating. If 
required, this sheet may be cut and shaped into a given size. 
According to one modification of a pair of toothed corrugating rolls, the 
female roll of said pair of toothed rolls, is provided with small holes in 
its troughs, so that the interior of its portions meshing with the male 
one is evacuated to vacuum. According to another modification, the male 
roll of said pair of toothed rolls, is provided with small holes, so that 
compressed air can be blown in the interior of the periphery of its 
portion meshing with the female one, thereby injecting said compressed air 
into said small holes. 
In order to make a corrugated cardboard including a corrugated core sheet 
having a plastic film laminated on the surface, that plastic film may be 
prelaminated on the primarily foamed plastic sheet. 
EXAMPLES 
Example 1 
The first method will now be explained in greater detail with reference to 
FIGS. 11 to 13. 
Referring to FIG. 11, a foamed polystyrene sheeting 16 in a rolled form, 
which has been primarily foamed to a foaming factor of 6 to 12 and is to 
be secondarily foamed, is fed into foaming equipment 10 by means of feed 
rolls 11 and 11', in which it is heated to an average resin temperature of 
120.degree. C. to subject it to secondary foaming simultaneously with 
forming skin layers on both its sides. After that, the thus foamed 
sheeting is passed between a pair of toothed rolls 12 and 12' regulated to 
an angle of about 40 to 60.degree. C. with a clearance therebetween, which 
is 0.1 to 2-mm smaller than the foamed sheet, thereby increasing the 
volumes of the skin layers. Then, the foamed polystyrene sheeting 16 is 
cooled on its both sides to around 40.degree. C. with air blowers 13 and 
13' to prepare a corrugated core sheet 2. 
This corrugated core sheet 2 is a corrugated, foamed polystyrene sheet 
having a closed-cell content of 70% or more, having on both sides the skin 
layers, each being 102 .mu.m in mean cell diameter in section and having a 
thickness of 350 .mu.m, with the core sheeting 16 being 226 .mu.m in mean 
cell diameter in section or having a sectional mean cell diameter of 1.8 
times or more as great as that of each skin layer. Then, the corrugated 
core sheet 2 receives a bonding agent--in an amount of 20 g/m.sup.2 
calculated as solid matter--for adhesive applicators 14 and 14', e.g. 
reverse roll coaters. In this case, the bonding agent may be spread all 
over the surfaces of paperboard liners 8 and 9, but this makes moisture 
permeability worse. Subsequently, the corrugated core sheet 2 is passed 
between two rubber rolls 15 and 15' at a pressure regulated to 1000 
g/cm.sup.2 or less, during which the paperboard liners 8 and 9 are guided, 
bonded and laminated onto the corrugated core sheet 2. If required, it may 
be advantageous to subject the liners 8 and 9 to preheating or premoisture 
conditioning. The corrugated core sheet 2 is then passed through driers 17 
and 17' regulated to normal temperature to 60.degree. C., preferably about 
40.degree. C. to complete bonding. Subsequently or if necessary, the sheet 
may be cut and shaped. The above-described operation was carried out at a 
speed of 12 m/minute. 
With the foaming machine 10 for secondary foaming, heating may be carried 
out in the following two manners. As shown in FIG. 12, a metal sheet 19 is 
uniformly heated by means of an infrared ray generator 18 to produce 
radiant heat, thereby heating the foamed polystyrene sheet 16 for its 
secondary foaming. Alternatively, as shown in FIG. 13, a far infrared ray 
generator 20 may be used to heat the foamed polystyrene for its secondary 
foaming. 
With the first-mentioned method, it is required to increase the surface 
temperature of resin to a level sufficient to heat the central region of 
the sheet 16, giving rise to a fear that the skin layers may decrease in 
volume. Thus, the second-mentioned method is preferable. In other words, 
it is preferable to offer control over the sheet 16 by measuring and 
regulating its surface temperature, thereby achieving uniform corrugation 
of the sheet and reducing shrinkage of the sheet as much as possible. It 
is noted that a foamed polyethylene sheet could be used in place of the 
foamed polystyrene sheet by increasing the processing temperature by about 
10.degree. C. 
Example 2 
This example is directed to the second method, which is virtually identical 
with the first method except that, as illustrated in FIG. 14, a primarily 
foamed polystyrene sheeting is first passed down through foaming equipment 
and then fed horizontally through the rest of the system operated in 
similar manners as described in Example 1. With the first method, it is 
unlikely that the polystyrene sheeting 16, when heated at the foaming 
step, may sag so that it comes into contact with the foaming equipment, 
thus offering some inconvenience. It is thus required to keep a sufficient 
distance between the underside of the sheeting 16 and the foaming unit, 
resulting in a difference between the two sides of the sheeting which is 
heated, or a drop in the efficiency of heating. With the second method 
designed to feed the sheeting vertically at the first step, however, such 
problems can be solved. 
More illustratively, a foamed polystyrene in a rolled form, which has been 
primarily foamed at a foaming factor of 5 to 10 and is to be secondarily 
foamed, is supplied down through a vertical type of foaming equipment 10, 
in which it is heated on its both sides to a resin temperature of 
120.degree. to 130.degree. C., thereby subjecting it to secondary foaming 
simultaneously with forming skin layers on both sides of the sheeting 16. 
After that, the sheeting 16 is maintained at 40.degree. to 60.degree. C. 
and then passed between metallic embossing rolls 21 and 21' for processing 
and diverting it. A pair of embossing rolls 21 and 21' are designed to be 
in light mesh with each other, as can be seen from a circle indicated by a 
dotted line, while the clearance is suitably regulated depending upon the 
thickness of the foamed polystyrene sheet to be processed. Once diverted 
horizontally, the sheeting is cooled by air blowers 13 and 13', receives a 
bonding agent from adhesive applicators 14 and 14' while paperboards 8 and 
9 are guided onto its both sides, is supplied between a metallic roll 15'' 
and a rubber roll 15' for press lamination, and is fed through a dryer 17 
for solidification and drying to obtain a corrugated cardboard product, as 
with the method described in Example 1. If required, the cardboard product 
may be cut and shaped. 
Example 3 
In this example, an embossing roll 21 for corrugation processing is 
provided with an array of holed bosses 22, on which vacuum acts to bring a 
foamed polystyrene sheeting 16 in close contact with them, thereby 
preventing its disengagement. The procedures of Example 2 were 
substantially followed with the exception that after leaving adhesive 
applicators, the feed was passed between two metallic rollers, thereby 
manufacturing a corrguated cardboard for keeping perishables, etc. fresh. 
This effect could be further increased by providing the associated 
embossing roll 21' with small holes and injecting compressed air from 
within the roll therethrough. 
It is understood that the shape retention of the polystyrene sheeting can 
be accelerated by the injection of precooled, compressed air. 
Example 4 
With such embossing equipment as shown in FIG. 16, a core sheeting was 
corrugated, and then processed to a corrugated cardboard in a batchwise 
fashion under the same conditions as described in Example 1. In FIG. 16, 
reference numeral 10 stands for foaming units, 23, a vacuum tank, 14, a 
male plug and 15, a female mold. 
Example 5 
A cardboard package (of 7 mm in thickness and with a core sheeting of 
foamed polystyrene and paperboard liners on its both sides) according to 
this invention and a general-purpose corrugated cardboard package, both 
packed with carnations produced in the northeastern section of Japan, were 
carried by a normal version of truck to Tokyo. 
The cardboard package according to this invention was prepared as follows. 
A primarily foamed polystyrene sheeting, which had a thickness of 1.6 mm 
and a mean foaming factor of 9, were provided with skin layers, each of 
120 to 200 .mu.m in thickness and 71 .mu.m in average cell diameter and 
included a core sheeting having a mean cell diameter of 174 .mu.m, was 
secondarily foamed and processed according to the first method into a 
corrugated sheet, which had a thickness of 3.1 mm, a closed-cell content 
of 90% or more and an average foaming factor 17, was provided with skin 
layers, each of 11 to 260 .mu.m in thickness and 102 .mu.m in average cell 
diameter and includes a core sheeting of 226 .mu.m in average cell 
diameter. Then, the core sheet was laminated on its both sides with 
paperboard liners into a corrugated cardboard sheet, which was in turn 
formed into a box package. 
The package, 800 mm long, 320 mm wide and 105 mm high, contained 3 kg of a 
coolant. Both the packages were unpacked eight hours after arrival. The 
flowering plant and leaves removed from the control package wilted to a 
considerable degree. The carnations removed from the inventive package, 
however, remained as lush and fresh as packed. 
Example 6 
Box packages were obtained using corrugated cardboard sheets, each 
including a core sheet laminated on both its sides with 25 .mu.m thick 
high-impact polystyrene films according to the procedures of Example 1. 
Frozen lobsters, 15 kg in total weight but 12 kg in net weight, were moved 
from a refrigerator warehouse into each of the packages, 64.5% in 
insulation effectiveness (measured according to ASTM method), 15.9 kgf/50 
mm (according to the JIS Z0401 residual vertical compression strength) and 
500 mm long.times.260 mm wide.times.215 mm high. The packages, along with 
control ones, were reciprocated between Tokyo and the Saitama district. 
The control packages were so embrittled due to water leakage that there 
was a serious degradation of freshness of the lobster. 
The inventive packages suffered no embrittlement. Nor did the underlying 
ones broke. Freshness of the lobsters was good. 
As a replacement to conventional packages formed of foamed polystyrene by 
injection molding, it has been attempted to make use of collapsible, 
corrugated cardboard packages laminated thereon with heat-insulating 
foamed plastic (PE, PS, PP, etc.) sheets, aluminium foils or aluminized 
(PET, PE, PP, etc.) films, thereby improving their heat insulation. 
However, they are still short of coming up to the heat insulation 
standards now demanded on the market. Thus, there is a strong demand 
toward developing packages in keeping with such standards. However, this 
demand will be satisfied by the present invention providing a package in 
which a skinned, foamed plastic sheet is used as the core sheet of a 
corrugated cardboard to improve its heat insulation to a much higher 
level. Further improvements in heat insulation is achieved by providing 
shielders on the core sheet. Thus, the present invention has succeeded in 
providing a corrugated cardboard package which is collapsible and nearly 
equivalent in heat insulation to foamed polystyrene injection moldings. 
According to this invention, it is also possible to prevent harmful gases 
or smells from entering packages and keep the in-package humidity at 
proper levels. It is understood that the packages according to this 
invention are superior in insulation effectiveness to aluminized, 
general-purpose corrugated cardboards, despite their limited wall 
thickness. 
Much more improvements in heat insulation and compression strength are 
achievable by laminating a plastic film or films on one or both sides of 
the corrugated core sheet according to this invention. This is 
particularly advantageous when a number of packages are to be stacked up. 
As detailed above, the keep-fresh, heat-insulating corrugated cardboard 
according to this invention has paperboard liners on its both sides and 
includes a corrugated core sheet formed of a foamed plastic sheet having a 
100 to 700 .mu.m thick skin layer formed on at least one side. Thus, it 
can be improved in heat insulation with no need of increasing the weight 
of materials. 
Using the foamed plastic sheeting as the corrugated core sheeting in place 
of conventional corrugated cardboard is economically so advantageous 
because of very expeditious proccessing of shielders to be provided in the 
core grooves, and contributes well to achieving high heat insulation. Such 
shielders assures much more improvements in heat insulation. In addition, 
laminating an additional foamed plastic sheet on one side of the 
corrugated core sheet through the paperboad liner is effective for 
preventing the surface of the contents from being scratched or scuffed. 
Thus, the present invention provides a collapsible and lightweight 
packaging corrugated cardboard, which has a great advantage of being much 
higher in wetting strength than conventional corrugated cardboard and 
excels in the performance of keeping perishables fresh. The present 
invention also provides a method capable of making products having such 
effects as mentioned above at low cost and in simple manners. Thus, the 
present invention makes a great breakthrough.