Sheet for producing plastic films and a process for producing this sheet

This invention concerns a sheet for producing plastic films with a tear-resistant layer and a plastic layer that is applied to the former and has a surface that faces away from the tear-resistant layer and is suitable for applying liquid or pasty plastics for the plastic film to be produced. The surface of the layer of plastic which is preferably a negative of a relief pattern consists of polypropylene. The sheet permits a high fidelity in reproduction of the relief pattern plus a long lifetime as well. The sheet may be relatively thin, so a great length can be wound up on a roll. Therefore, great lengths of a plastic film can be produced in one piece by casting or spreading liquid plastic on the sheet. Therefore, the conversion time required for the manufacturing installation is short, which has a positive effect on the price of the product.

BACKGROUND OF THE INVENTION 
This invention concerns a sheet of the type defined in the generic portion 
of claim 1. This invention also concerns a process for producing such a 
sheet. 
It is known that a plastic film can be produced by applying a thin layer of 
a liquid or pasty plastic to a substrate, then curing the plastic and next 
pulling away the layer in the form of a film. This is usually accomplished 
in a continuous process, where the substrate is a continuous or even a 
finite backing sheet that has either a smooth surface or an embossed 
relief pattern. 
A known sheet of this type is made of paper embossed with a relief pattern 
that extends to the back side of the paper, so the cross section shows a 
corrugated fine structure that corresponds to the embossed pattern. To 
improve the surface properties, especially in order to facilitate 
separation of the plastic compound applied or spread on the paper 
subsequently as a film, the embossed paper is provided with a thin layer 
of plastic after embossing. 
This known sheeting is simple and therefore inexpensive to manufacture, but 
it has several disadvantages. One disadvantage is that it has a low 
resistance to tearing. In practice, tears develop quickly, especially at 
the edges, and can lead to rejects in the production of the plastic 
sheeting and can even lead to tearing of the sheet itself, which causes 
interruptions in production and, if repairs are made, this leads to 
discontinuities in the relief pattern, so the repair is visible. 
Another disadvantage of this known sheet is that only a shallow relief 
pattern, such as a grain, can be embossed, so this permits only incomplete 
reproduction of the desired relief pattern. In addition, another 
disadvantage is that the force required to separate the sheet-from the 
backing increases as the sheet is used repeatedly, so each sheet has only 
a limited lifetime. The number of repeated uses of this known sheet is 
therefore low, so despite the low manufacturing cost of this sheet, the 
total cost of the plastic film produced with this sheeting is further 
increased, so the advantage of the low price of the sheeting is lost. 
Another disadvantage of this known and widely used sheeting consists of its 
low dimensional stability and its ability to absorb moisture, which has a 
negative effect on its strength, dimensional stability and handling in 
practice. 
Austrian patent 262,594 discloses a sheet of the respective type that has a 
tear-resistant layer consisting of a fabric, for example, to which a layer 
of polysiloxane rubber with a relief pattern embossed on the surface is 
applied on one side. The polysiloxane rubber (silicone rubber) permits an 
accurate reproduction of a fine surface structure, such as a leather 
grain, but this sheeting has several disadvantages. One disadvantage is 
that the price is much higher--for example, 10 times higher--than the 
price of the paper sheeting described above. That does not compensate for 
the advantage that the lifetime of such silicone sheeting is approximately 
four times longer than that of the paper sheeting described above. In 
addition, the polysiloxane rubber layer is relatively thick due to the 
production process. Therefore, there are disadvantages in production when 
such sheeting is used in the form of finite sheets in production rather 
than continuous sheeting. The sheeting is unwound from a large roll, 
passes through the casting and curing line and then is rolled up again 
after removing the plastic film produced in this process. This yields 
relatively large rolls or short lengths of sheeting that lead to frequent 
interruptions in the manufacturing process. 
An older unpublished German patent application P 43 22 140.8 discloses a 
plastic film based on polymers of propylene and ethylene which may 
optionally also contain processing aids, fillers, pigments or other 
additives. It contains a) a homopolymer of propylene with an MFI 
(230.degree. C./2.16 kg) of about 0.8 to 3.0 g/10 min, a melting point 
T.sub.m, of about 154.degree. to 168.degree. C. (determined by DSC) and an 
elastic modulus of about 900 to 1500 N/mm.sup.2 (determined according to 
DIN 53,457) and b) a homopolymer of ethylene with an MFI (190.degree. 
C./2.16 kg) of about 1.5 to 3.0 g/10 min, a melting point T.sub.M, of 
about 110.degree. to 130.degree. C. and an elastic modulus of about 200 to 
400 N/mm.sup.2. However, this plastic film is not directly suitable for 
production of plastic films by applying a liquid or pasty plastic. 
SUMMARY OF THE INVENTION 
The object of this invention is to create sheeting of the proper type for 
producing plastic films with a surface having a very fine structure and a 
high fidelity of reproduction. The sheeting should also be inexpensive and 
easy to handle and should permit long production times. 
The object of this invention is achieved by a sheeting consisting of a 
tear-resistant layer with a plastic layer bonded to it, where the surface 
of the plastic layer facing away from the tear-resistant layer is suitable 
for applying liquid or pasty plastics for the plastic film to be produced, 
wherein the plastic layer with the surface consists at least partially of 
polypropylene.

DETAILED DESCRIPTION OF THE INVENTION 
The basic idea on which this invention is based consists of using a plastic 
that contains polypropylene for the surface layer instead of using the 
known polysiloxane rubber. It has surprisingly been found that the 
contours of the surface, regardless of whether the surface is smooth or 
has the desired relief pattern according to another embodiment of this 
invention, can be reproduced with good fidelity, even if the thickness of 
the layer of the plastic is small, or at any rate much smaller than the 
thickness of the layer of polysiloxane rubber with the known sheeting. In 
addition, the lifetime of the sheeting according to this invention is much 
longer, amounting to about three times that of the known polysiloxane 
rubber sheeting and about fifteen times that of the known paper sheeting. 
Other advantages consist of its high dimensional stability and its 
stability in the presence of aqueous solutions, plus the fact that the 
layer of plastic can be extremely thin. Therefore, a much greater length 
can be wound onto a roll of this sheeting. This in turn yields much longer 
running times in production of such plastic sheeting and the equipment 
need not be retooled as frequently, which has a positive effect on the 
final price of the finished plastic film. The polypropylene layer has a 
non-stick effect, so no additional parting compounds are necessary to be 
able to pull a fully cured plastic film away from the sheeting according 
to this invention. The sheeting according to this invention also has a 
high mechanical strength, especially with respect to tears at the sides, 
which occur after a very short period of time with the known paper 
sheeting. Finally, an advantage of the sheeting according to this 
invention is that unlike the known polysiloxane rubber sheeting, the 
surface of this sheeting, especially a relief pattern in the surface, can 
be renewed very easily by embossing it again with heat thanks to the 
thermoplastic workability of the plastic. 
According to one embodiment of this invention, the tear-resistant layer of 
the sheeting is made of a fabric, and it is especially expedient to use 
glass fiber weave. 
According to another embodiment of this invention, the plastic also 
contains talc as a filler. This improves both the dimensional stability 
and the lifetime of the polypropylene as well as the relief pattern 
embossed in the plastic. 
Finally, another embodiment of this invention consists of the fact that the 
plastic contains some polyethylene in addition to the polypropylene, where 
the amount of polyethylene is preferably about 20% by weight. 
Another object of this invention is to create a process for producing 
sheeting of the type consisting of a tear-resistant layer with a plastic 
layer bonded to it, where the surface of the plastic layer facing away 
from the tear-resistant layer is suitable or applying liquid or pasty 
plastics for the plastic film to be produced, wherein the plastic layer 
with the surface consists at least partially of polypropylene. It consists 
of the fact that a layer of polypropylene is applied to a tear-resistant 
sheeting. This of course includes the fact that the layer of polypropylene 
may be applied in the form of a film, for example, to the tear-resistant 
sheeting. The polypropylene layer is then heated to the softening point, 
the free surface of this layer is then embossed by means of an embossing 
roll having a positive image of the desired relief pattern in its 
peripheral surface, after which the polypropylene layer or the entire 
sheeting is cooled, so then the relief pattern is permanently embossed in 
the surface of the polypropylene layer. It is especially advantageous if 
the polypropylene layer is heated to the melting point. This layer is then 
capable of penetrating into fine structures of a relief pattern and 
reproducing it faithfully, which is especially important when the model 
pattern is leather or a similar material. 
An embossing roll with an embossing surface made of steel or preferably 
polysiloxane rubber may be used to emboss the softened or molten surface. 
It is also expedient to provide the polypropylene layer with a primer 
before bonding it to the tear-resistant sheeting. 
If the plastic layer contains polyethylene in addition to polypropylene, it 
preferably contains a) a homopolymer of propylene with an MFI (230.degree. 
C./2.16 kg) of about 0.8 to 3.0 g/10 min, a melting point T.sub.m of about 
154.degree.-168.degree. C. (determined by DSC) and an elastic modulus of 
about 900 to 1500 N/mm.sup.2 (determined according to DIN 53,457), and b) 
a homopolymer of ethylene with an MFI (190.degree. C./2.16 kg) of about 
1.5 to 3.0 g/10 min, a melting point T.sub.m of about 
110.degree.-130.degree. C. and an elastic modulus of about 200 to 400 
N/mm.sup.2, where the resulting ratio is approximately 5 to 20 parts by 
weight of the ethylene homopolymer to about 100 parts by weight of the 
propylene homopolymer. 
The abbreviation DSC refers to "differential scanning calorimetry" and is 
described in detail in Marti et al., Applied Chemical Thermdynamics and 
Thermal Analysis (Experimental Suppl. 37), Basel, Birkhauser, 1979. 
A special embodiment of this plastic with regard to the physical values 
given above contains a homopolymer of propylene with an MFI (230.degree. 
C./2.16 Kg) of about 1 to 2 g/10 min, a melting point T.sub.m of about 
158.degree.-164.degree. C. and an elastic modulus of about 1100 to 1300 
N/mm.sup.2, especially in the form of a CR product, and a homopolymer of 
ethylene with an MFI (190.degree. C./2.16 kg) of about 1 to 2 g/10 min, a 
melting point T.sub.m of about 110.degree. to 120.degree. C. and an 
elastic modulus of about 250 to 350 N/mm.sup.2. 
The homopolymer of propylene preferably has a narrow molecular weight 
distribution. Such products are available commercially as CR products 
(CR=controlled rheology). An especially suitable homopolymer of ethylene 
is a linear low-density ethylene homopolymer (LLDPE) or a low-density 
polyethylene produced under low pressure (LDPE). The term "homopolymer" 
means that this polymer consists essentially of ethylene or propylene, 
respectively. In no case does this refer to a copolymer. Small amounts of 
other monomers may thus be included. In general, the term homopolymer in 
the sense of the present invention would also include a polymer that 
contains up to about 10 mol % of one or more comonomers, especially in the 
form of isoprene, 1,3-butadiene, ethylene (in the case of a homopolymer of 
propylene), butene, hexene and octene or the isomers thereof, in addition 
to ethylene or propylene. 
The properties of the plastic forming the surface may be modified through 
the use of additives that are incorporated into the thermoplastic. Thus, 
for example, fillers such as metal oxides, metal carbonates, especially 
calcium carbonates and dolomite, metal dioxides and metal hydroxides, 
lubricants such as C.sub.12 -C.sub.16 fatty acids, fatty alcohols, fatty 
acid esters and amides, coloring agents in the form of pigments such as 
titanium dioxide or carbon black and organic pigments, stabilizers such as 
antioxidants and heat stabilizers such as sterically hindered phenols, 
hydroquinones, substituted representatives of this group and mixtures 
thereof, UV stabilizers such as various substituted resorcinols, 
salicylates, benzotriazoles and benzophenones as well as other 
conventional modifiers may also be used. Other suitable additives include 
processing aids such as high-molecular acrylates or acrylate copolymers. 
They control the flow properties of the melting process during processing. 
Another especially advantageous embodiment of the plastic surface would 
also contain an ethylene-propylene copolymer, with about 5 to 40 parts by 
weight, especially about 10 to 25 parts by weight ethylene-propylene 
copolymer to 100 parts by weight propylene homopolymer. It is preferable 
here for the ethylene-propylene copolymer to have an MFI (230.degree. 
C./2.16 kg) of about 1 to 4 g/10 min, a melting point T.sub.m of about 
140.degree. to 155.degree. C. and an elastic modulus of about 400 to 700 
N/mm.sup.2.