The invention relates to a multi-layer product, in particular for producing containers, comprising a layer formed of a fiber-based packing material on one surface of which a gas-tight multi-layer plastic coating is arranged. To prevent the formation of microholes, the gas-tight multi-layer plastic coating is formed of superposed 1-4 g/m.sup.2 barrier plastic layer, 1-4 g/m.sup.2 binder layer, and surface layer of a heat-sealable polyolefin material having sufficient thickness for liquid-tight heat sealing, so that the barrier plastic layer is the closest of said layers to the fiber-based packing material layer.

BACKGROUND OF THE INVENTION 
The present invention relates to a multi-layer product, in particular for 
producing containers, comprising a layer formed of a fiber-based packing 
material on one surface of which a gas-tight multi-layer plastic coating 
is arranged. 
Multi-layer products of the above type are today very well known for 
example in connection with containers for citrus fruit juices. Frequently, 
the fiber-based material is cardboard. The solution disclosed in European 
published application 0 293 098 is an example of the prior art solutions. 
In connection with the prior art solutions, problems have been encountered 
in the manufacturing step of containers, particularly in heat sealing the 
plastic surfaces together, wherein pinholes may be induced in the 
multi-layer plastic coating. Pinholing presents a particular problem when 
the holes are produced on the inner surface of the container. The good 
gas-tightness characteristics of the plastic coating are in that case 
partly lost, since gas flow may even occur through microholes, even though 
despite the microholes the coating were liquid-tight. 
Pinholing of the plastic coating is a result of, among other things, the 
fact that the high temperature applied to the coating in heat sealing 
softens and partly melts the plastic layers. A further reason for 
pinholing is that also the cardboard is heated in the sealing region, and 
on account of steam pressure the moisture in the cardboard tends to 
penetrate the softened and partly molten plastic layers, thus producing 
pinholes in the coating. The fact is that the higher the heat sealing 
temperature and the higher the moisture content in the cardboard, the more 
readily pinholes will be induced. The steam pressure in the cardboard 
tends to burst, spot-wise at small points, the plastic layer applied to 
the cardboard. The burst point serves as an initiator for pinholing. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a multi-layer product 
wherewith the drawbacks of the prior art can be eliminated. It has been 
found in connection with the invention that by selecting the type of the 
heat-sealable plastic for the multi-layer structure so that the heat 
sealing can be performed at a low temperature, pinholing can be diminished 
and even totally prevented. Selection of the plastic layer in accordance 
with the above results in that the plastic layers are not softened and do 
not lose their strength as readily as in the earlier solutions, thus 
hindering pinholing. On the other hand, the steam pressure generated by 
moisture in the cardboard is diminished. Pinholing can also be prevented 
by selecting as the plastic layer to be applied to the cardboard a plastic 
type that remains ductile and strong at heat sealing temperatures. 
Pinholing can further be influenced by selecting a plastic type that 
remains ductile in heat sealing for one of the layers. The multi-layer 
product of the invention is characterized in that the gas-tight 
multi-layer plastic coating is formed of superposed 1-4 g/m.sup.2 barrier 
plastic layer, 1-4 g/m.sup.2 binder layer, and surface layer of a 
heat-sealable polyolefin material having sufficient thickness for 
liquid-tight heat sealing, so that the barrier plastic layer is the 
closest of said layers to the fiber-based packing material layer. 
The advantage of the invention over the prior art is first and foremost the 
fact that a higher strength and ductility than heretofore is achieved for 
the multi-layer product, and these properties are retained also at high 
temperatures, such as those employed in heat sealing. Consequently, the 
invention can employ even 15-30% lower coating thicknesses than 
heretofore. Furthermore, it is essential that the hot tack and strength of 
the seal in the heat sealing are improved, thus allowing the line speed in 
heat sealing to be increased by 10-20% with the same temperatures and 
quantities of heat. This factor again diminishes pinholing in the plastic 
coating. A further advantage of the multi-layer product of the invention 
is that the environmental stress crack resistance ESCR is much better than 
heretofore. This characteristic is significant in packaging of 
hard-to-hold products. For example in containers for liquid detergents, a 
high ESCR enables reduction of the total thickness of the plastic coating 
approximately by 30%, if one aims at a storage time of about one year. A 
further advantage of the invention is high abrasion resistance. As a 
result, no plastic dust is produced on high-speed container manufacturing 
lines, and the blanks run more smoothly on the lines than heretofore. In 
this connection it may be mentioned by way of example that when for 
instance a linear low-density polyethylene, i.e. a PE-LLD material, is 
used as the surface layer of the multi-layer plastic coating, the abrasion 
resistance is approximately twice the abrasion resistance of the prior art 
products. Also the other advantages set forth above have been explained 
mainly with PE-LLD material being the surface layer material in said 
multi-layer plastic coating.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows a first preferred embodiment of the multi-layer product of the 
invention. Reference numeral 1 in FIG. 1 denotes a fiber-based packing 
material, which may be for example cardboard. In this embodiment, a layer 
2 of a low-density polyethylene (referred to in this specification and in 
the drawings as PE-LD) is provided on one surface of the cardboard. The 
layer 2 is intended to provide the outer surface for the finished 
container. The PE-LD layer 2 is not indispensable, as the outer surface of 
the finished container can be realized in other ways as well. A lacquer 
surface or a surface otherwise treated to be water-repellent, or in some 
cases even an untreated cardboard surface, may be mentioned as examples. 
In this embodiment, a gas-tight multi-layer plastic coating 3 is applied to 
the other surface of the cardboard 1, that is, to that surface which is 
intended to provide the inner surface of the finished container. The 
multi-layer plastic coating 3 is formed of superposed barrier plastic 
layer 4a, binder layer 5, and surface layer 6 of a heat-sealable 
polyolefin material. 
In the embodiment of FIG. 1, the barrier plastic layer 4a is made of a 
ethylene vinyl alcohol copolymer containing 32 mole percent ethylene, and 
the surface layer 6 providing the inner surface of the container in turn 
is made of linear low-density polyethylene, (referred to in this 
specification and in the drawings as PE-LLD). The binder layer 5 can be 
for instance acid-modified PE-LLD. In the embodiment of FIG. 1, the layer 
thicknesses of the different layers in the multi-layer plastic coating are 
as follows: barrier plastic layer 4a 1-4 g/m.sup.2, binder layer 5 1-4 
g/m.sup.2, and the thickness of the surface layer 6 is chosen to be 
sufficient for liquid-tight heat sealing. The thickness of layer 6 can 
vary depending on the product to be packed, for instance to be less than 
20 g/m.sup.2 with juices and to be in excess of 20 g/m.sup.2 with 
aggressive substances, such as detergents. The thickness of the PE-LD 
layer 2 on the reverse side of the cardboard can be for example 20 
g/m.sup.2. 
With the above solution, a thin, more advantageous coating and better 
barrier properties than heretofore are obtained. The further advantages of 
the solution according to FIG. 1 include good mechanical abrasion 
resistance, advantageous ESCR, good heat sealability, a lower sealing 
temperature, and good after-treatment characteristics. The embodiment 
according to FIG. 1 can also be used with advantage in connection with 
hard-to-hold liquids. 
FIG. 2 shows another preferred embodiment of the invention. The embodiment 
of FIG. 2 essentially corresponds to the embodiment of FIG. 1, and 
therefore FIG. 2 employs the same reference numerals as FIG. 1 at 
corresponding points. The only difference to the example according to FIG. 
1 is that the barrier plastic layer 4b of the gas-tight multi-layer 
plastic coating 3 is made of a blend of an ethylene vinyl alcohol 
copolymer (referred to in this specification and in the drawings as EVOH) 
and mica. This blend is a substance known per se, and is more closely 
described for instance in U.S. Pat. No. 4,818,782. 
Mica gives EVOH good adhesion to cardboard in extrusion coating, and good 
adhesion in turn diminishes the tendency to blowing. Further, mica adds to 
the strength of EVOH, and this added strength is especially important in 
connection with heat sealing, thereby diminishing the tendency to blowing. 
Mica is also a material having a higher heat conductivity than EVOH, and 
thus the sealing heat is not as readily stored in the EVOH. 
Mica imparts to EVOH good gas barrier properties and serves to reduce 
permeability to ultraviolet light. In view of recycling, the combination 
of ethylene vinyl alcohol copolymer and mica (referred to in this 
specification and in the drawings as EVOH+mica) is more practical than 
EVOH alone. 
On account of the mica, the heat sealing temperature can be maintained at 
the same level in the blank production and on the filling machine as with 
the use of Al-foil cardboard. Likewise, the operating window for heat 
sealing is equally wide in the case of EVOH+mica as when Al-laminated 
cardboard is used. Without mica, the heat sealing temperature must be 
reduced by about 30.degree. C. and the operating window diminished by 
about 20.degree. C. When mica is added, however, the process can best be 
realized at the lower end of the operating window, and thus the steam 
pressure from the water contained in the cardboard is low and no 
microholes are induced. With no mica, the occurrence of microholes would 
be very likely. 
FIG. 3 shows a third preferred embodiment of the invention. The embodiment 
of FIG. 3 substantially corresponds to the embodiments of FIGS. 1 and 2. 
The only difference is that in the embodiment of FIG. 3, the barrier 
plastic layer 4c is made of a blend of ethylene vinyl alcohol copolymer 
and a copolymer of ethylene and carbon monoxide (ECO). FIG. 3 employs the 
same reference numerals as FIGS. 1 and 2 at corresponding points. 
As stated previously, in the embodiment according to FIG. 3 the barrier 
plastic layer 4c is made of a blend of ethylene vinyl alcohol copolymer 
and an ethylene polymer having C.dbd.O groups, (referred to in the 
drawings as EVOH+ECO). Ethylene vinyl alcohol copolymer containing an 
ethylene polymer having C.dbd.O groups has higher melt strength than EVOH 
plastic alone. High melt strength is an important factor in view of 
extrusion coating. The inferior melt strength of neat EVOH causes 
pinholing in the EVOH layer in the extrusion coating alone. This is a 
result of the fact that the fibers of the cardboard to be coated tend to 
rupture a weak molten EVOH layer. Addition of for instance 30% by weight 
of ethylene polymer having C.dbd.O groups to the EVOH improves the melt 
strength so significantly that the fibers do not induce pinholes in the 
molten plastic film. This for its part enables direct application of 
EVOH+ECO to cardboard. 
High melt strength prevents bursting of EVOH+ECO plastic in heat sealing 
and thereby precludes microhole formation. Generally speaking, an EVOH+ECO 
plastic layer remains very strong and ductile in heat sealing, and thereby 
the bursting tendency is substantially lower than with EVOH plastic alone. 
Moreover, EVOH+ECO is less sensitive to variations in ambient humidity than 
EVOH alone. This is reflected on the heat sealing characteristics so that 
the steam pressure from the cardboard does not impair the strength of the 
blend to an equal extent as with neat EVOH. On the other hand, reduction 
in sensitivity to moisture is also reflected as a distinct decrease in the 
effect of ambient humidity on gas tightness. 
FIG. 4 shows a fourth preferred embodiment of the invention. The embodiment 
of FIG. 4 substantially corresponds to the embodiments of FIGS. 1-3. An 
essential difference to the previous embodiments is that the barrier 
plastic layer 4d is made of a blend of an ethylene vinyl alcohol copolymer 
and polyamide. FIG. 4 employs the same reference numerals as the 
embodiments of FIG. 1-3 at corresponding points. 
In the embodiment of FIG. 4, the barrier layer is a blend of ethylene vinyl 
alcohol copolymer and polyamide (referred to in this specification and in 
the drawings as EVOH+PA). Various blends of EVOH and polyamide (PA) are 
known per se; examples of these are U.S. Pat. Nos. 4,952,628, 5,110,855 
and 5,126,401. PA plastic is a very ductile and strong plastic. PA has a 
melting point between 230 and 240.degree. C., while the melting point of 
EVOH is 168.degree. C. The gas tightness characteristics of PA are not 
equal to those of EVOH. By blending EVOH with PA, the following advantages 
are achieved in multi-layer applications. Strength and ductility are 
improved in extrusion coating and heat sealing. For this reason, no 
pinholing occurs in extrusion coating, and, on the other hand, on account 
of the ductility the EVOH+PA layer will not rupture or let steam pressure 
through. In heat sealing, the operating window is 30.degree. C. broader 
than with EVOH alone. In view of pulping, neat PA is a plastic that is too 
ductile. EVOH+PA in an appropriate ratio is a suitable solution in view of 
pulping. 
FIG. 5 discloses a fifth preferred embodiment of the invention. Also this 
embodiment substantially corresponds to the previous examples. The 
difference to the previous examples is that in this example the barrier 
plastic layer 4e is made of a thermoplastic polyester or thermoplastic 
copolyester. Examples of such materials are polyethylene terephthalate and 
polybutylene terephthalate. FIG. 5 employs the same reference numerals as 
the previous embodiments at corresponding points. 
In the embodiment of FIG. 5, the barrier plastic is polyethylene 
terephthalate (referred to in this specification and in the drawings as 
PET). The gas tightness characteristics of PET are not equal to those of 
EVOH or PA, but it is significant that the barrier properties of PET do 
not vary depending on ambient humidity. The heat resistance 
characteristics of PET are good; the melting point is 255.degree. C. PET 
also has good strength characteristics and remains ductile at temperatures 
exceeding 200.degree. C. Thus microholes are not induced in PET in 
connection with heat sealing, and furthermore pinholing will not occur in 
connection with extrusion coating. PET plastics have sufficient adhesion 
to cardboard only when the amount of PET coating is about 40 g/m.sup.2 or 
more. By using co-extrusion coating to produce for example a 3-layer 
coating, adhesion can however be accomplished even with small amounts of 
PET coating. All coating amounts mentioned in connection with FIG. 1 are 
also suitable for the embodiments of FIGS. 2 to 5. 
A further property of PET plastics is that the variation of melt viscosity 
can even cause marked instability in extrusion coating. In three-layer 
extrusion coating, the instability of PET plastics is compensated for with 
the good stability of the other two layers. Good stability corrects 
grammage variations, edge waving in the plastic coating vanishes, and 
these factors together render the use of PET production-economical. 
Further, since PET plastic is stiff, it improves the rigidity of the 
container and enables long-term storage without bulging. This fact is 
essential particularly in connection with aseptic containers. 
In all of the above embodiments, the barrier plastic layer is applied 
immediately to the fiber-based packing material layer 1. Such an 
arrangement is particularly advantageous when the surface of the 
fiber-based packing material layer is sufficiently smooth. An example of 
such a material is hot-calendered cardboard. If the surface of the 
cardboard is rough, it is preferred to apply a smoothing layer to the 
surface of the cardboard, smoothing the irregularities of the surface of 
the cardboard, in which situation the desired effect is achieved with a 
barrier plastic layer of a preferred thickness. The upright fibers in the 
rough surface may easily extend through the barrier plastic layer, thus 
impairing the efficiency of the finished product. The purpose of the 
smoothing layer is to provide the desired efficiency without any need for 
inordinately increasing the thickness of the barrier plastic layer. 
FIG. 6 shows an embodiment based on the embodiment of FIG. 1. The 
embodiment of FIG. 6 employs a smoothing layer 7 between the barrier 
plastic layer 4a and the fiber-based packing material layer 1. Further, 
the embodiment of FIG. 6 has no PE-LD layer on the other surface of layer 
1, but for instance a lacquer layer is applied to said surface, as set 
forth above. The lacquer layer is however not shown in FIG. 6. It is 
obvious that the embodiment according to FIG. 6 can also be formed of the 
embodiments of FIGS. 2-5. It is also obvious that a PE-LD layer to be 
applied to the outer layer of the finished container can be used in 
connection with the embodiment employing a smoothing layer 7. The 
smoothing layer 7 can preferably be made of the same material as the 
binder layer 5. The thickness of the smoothing layer 7 can advantageously 
be of the same order as the thickness of the binder layer, for example 
less than 3 g/m.sup.2. 
The multi-layer structure of the invention can be manufactured for instance 
in the following manner. The manufacture will be explained below by means 
of the embodiment of FIG. 2, but it is obvious that all of the embodiments 
can be manufactured similarly. A web of cardboard 1 to be coated can, if 
necessary, be treated by the flame, corona, primer, or plasma method on 
both sides. The pretreated cardboard or paper is coated on both sides in a 
single run as follows: The PE-LD coating 2 of the surface providing the 
outer surface for the finished container is performed on flamed cardboard 
by the extrusion method. The plastic coating 3 of the inner surface of the 
finished container is performed in a single run by extruding all necessary 
layers at one time to provide the 3-layer coating shown in the figure. 
This enables low extrusion temperatures to be used, so that the 
heat-sensitive EVOH will not be thermally degraded. EVOH+mica is fed from 
an extruder of its own, the binder polymer from an extruder of its own, 
and PE-LLD from an extruder of its own. The outer surface of the cardboard 
laminate coated in the above manner is corona treated to enable printer's 
ink to strike in and to improve heat sealing. Also other treatments 
enhancing printing, such as Printable Glueable (PG) treatment, are 
possible. The cardboard-plastic laminate thus prepared can now be heat 
sealed by conventional methods. 
The invention set out above is particularly advantageous as a packing 
material for citrus juices, water, and other fluid foodstuffs, for example 
materials that are folded into square, rectangular or cylindrical 
containers. 
The invention disclosed above has in no way been limited to the embodiments 
presented, but the invention may be modified fully freely within the scope 
of the claims. Thus it is obvious that the multi-layer product of the 
invention or its details need not necessarily be exactly as shown in the 
figures, but other solutions are possible as well. For example, layer 6 is 
not limited to PE-LLD material, but the polyolefin of said layer may be 
for example low-density polyethylene, high-density polyethylene, 
polypropylene, polybutylene, etc. The polyolefin material of layer 6 may 
also be a blend, for example a blend of a linear low-density polyethylene 
and a low-density polyethylene. The fiber-based packing material need not 
necessarily be cardboard, but other materials are possible as well, such 
as paper, plastic-coated cardboard, etc.