EP0585585A1 - Photographic support with a polyolefine mixture as backside coating - Google Patents

Abstract

The photographic support has a back coating, which consists predominantly of a mixture of low-density polyethylene and polypropylene.

Description

The invention relates to a photographic support with synthetic resin layers applied on both sides.

Photographic substrates mainly consist of photographic base papers with synthetic resin coatings. These synthetic resin coatings are mostly applied on both sides and consist of a clear, unpigmented polyolefin layer on the back of the base paper and a polyolefin layer filled with white pigments on the front. The front is the side that will later carry the light-sensitive photographic emulsions or the developed image.

The flatness of such a layer support is initially ensured when the front and back coatings are approximately identical.

However, since only the front is coated with light-sensitive photographic emulsions in a later step, this balance is no longer given. In order to compensate for the effect of this additional front-side coating, the back-side coating is therefore already reinforced during the production of the layer support in such a way that the later overall product again has good flatness. This can be done by an increased layer thickness of the backside coating, by a coating material that has increased tensile forces or higher rigidity or by additionally applied so-called anti-curl layers.

In photographic substrates, the synthetic resin is usually a polyethylene. In order to ensure the flatness of the end product, an approximately 30-35% higher proportion of HDPE (high density polyethylene) is added to the back coating than the front coating. In contrast to LDPE (low density polyethylene), HDPE is of greater crystallinity, contains increased amounts of stabilizing agents and residues of catalysts and acid acceptors, and contains more areas of high molecular weight enrichments (gel bodies). The gel bodies are noticeable as non-melting areas. With higher HDPE contents in the coating mass, this leads to difficult extrusion conditions due to faster pressure build-up in the extruder, caused by increased residues on the screens and increased reject material due to gel bodies (high molecular areas that cannot be broken down in the melt) in the coating. The greater crystallinity and the increased content of stabilizing agents make it difficult to adhere to the base paper.

One way to avoid curl or to ensure that the photographic material is sufficiently flat is described in DOS 17 72 347. In Example 5 of this document, the front coating consists of polyethylene plus white pigment and the back coating
made of polypropylene. The processing conditions for polypropylene in the melt extruder differ from those for polyethylene z. B. in melting temperature and pressure and even, should be run under optimal conditions, in a changed geometry of the extruder screw. If the extruders are used for both polyolefins, time losses must therefore always be planned for the changes. Polypropylene also adheres poorly to paper surfaces than LDPE.

Such adhesion problems are said to be solved in WO 90-002 973 by coextrusion. In this patent, the back coating of the base paper consists of at least 2 layers, an adhesive layer and a layer, e.g. B. made of polypropylene, to adjust the curl behavior. However, this type of coating requires the higher cost of coextrusion.

It is therefore an object of the invention to develop a back coating by the melt extrusion process which ensures the flatness of the photographic support after emulsioning or imaging, without having to accept difficult or more complex production conditions, less layer adhesion or increased gel bodies in the coating .

The problem was solved by a photographic support consisting of a support and synthetic resin layers applied on both sides, the synthetic resin layer on the back entirely or predominantly from a mixture of low-density polyethylene (LDPE) and polypropylene homopolymer and / or polypropylene copolymer of at least 80% by weight. -% propylene and at most 20% by weight of at least one other olefin.

The low-density polyethylene used in the mixture can also be a linear polyethylene (LLDPE) and / or a polyethylene / (C₃ - C₈) -α-olefin copolymer.

The polypropylene homopolymer and the polypropylene copolymer can consist of 80-100% by weight of propylene and 0-20% by weight.
(C₂ to C₄) olefin, preferably ethylene, be composed.

, was sich in verbesserter Fließfähigkeit, Ausziehfähigkeit und in geringer Wasserdampfdurchlässigkeit auswirkt.
   M_w = durchschnittliches Molekulargewicht
   M_n = durchschnittliche Molekulargewichts-Anzahl
Die Mischung kann bis zu 20 Gew.-% andere Polyolefine, z. B. hochdichtes Polyethylen (HDPE), enthalten.Polypropylene homopolymers and polypropylene copolymers with a narrow molar mass distribution of are particularly preferred
${\text{M}}_{\text{w}}{\text{/ M}}_{\text{n}}\text{= <6}$

, which results in improved fluidity, extensibility and low water vapor permeability.
M _w = average molecular weight
M _n = average number of molecular weights
The mixture can contain up to 20% by weight of other polyolefins, e.g. B. high density polyethylene (HDPE).

The mixture can also contain up to a total of 20% by weight of white pigments (e.g. titanium dioxide), color pigments (e.g. ultramarine blue), inorganic and organic fillers (e.g. silicon dioxide), optical brighteners (e.g. stilbene derivatives), lubricants (e.g. waxes), release agents ( contain fluoropolymers), antistatic agents (e.g. ethoxylated tertiary amines) and other common auxiliaries (e.g. antioxidants and anti-aging agents).

The amount of polypropylene homopolymer and / or polypropylene copolymer in the mixture according to the invention is 2 to 75% by weight.

Low density polyethylene (LDPE) is polyethylene with a specific weight of 0.91 - 0.94 g / cm³.
High-density polyethylene (HDPE) is considered to be polyethylene with a specific weight of 0.94 - 0.97 g / cm³.

The mixture according to the invention can be processed without problems using the extruder screws, which are also intended for pure polyethylene.

Due to the complete or partial replacement of HDPE with polypropylene homopolymer or copolymer, the gel body level in the coating is eliminated or reduced to tolerable values.

The amount of polypropylene homopolymer or copolymer that is really necessary for the flatness of the photographic support or the image-bearing end product (image support material) depends on the proportion of HDPE or other polyolefins in the front and back coating and the difference in the application weights between the front and back coating.

The synthetic resin layers are produced by the melt extrusion process and applied to a carrier, in order to improve the adhesion of the layers on the carrier, preferably a surface activation, e.g. a corona pretreatment.

Plastic films, textiles and papers are suitable as carriers. The plastic films are mostly polyester films, the papers mostly photographic base papers. The photographic base papers are a preferred support.

The photographic layer support produced in this way can carry further customary auxiliary or functional layers such as primer or adhesive layers, antistatic, anticurl or stiffening layers or writable and printable layers.

The following examples are intended to illustrate but not limit the invention:

Examples

A neutral sized photographic base paper of 175 g / m² basis weight was successively coated using a melt extrusion process as follows.

First the back of the base paper was coated after a corona pretreatment, then the front of the base paper after a corona pretreatment.

The machine speed in the extruder was 150 m / min, the melting temperatures were between 290 ° C and 320 ° C.

The following Table 1 lists the polyolefin mixtures and the application weights of the coatings:

Test results

The curl is measured on circular specimens with a diameter of 17 cm. The panes are kept in a climatic chamber for 4 or 14 days at 80% relative humidity and 23 ° C.

The curl is measured as follows:
The curl to the front is given a plus (+)
The back of the curl is given a minus (-) sign
It is believed that the curl of the punched-out disk describes an arc. This assumed full circle is divided into eighths.

The curl is expressed in eighths, e.g. B.
a curl of 8 would be 8/8 = 1 full circle
a curl of 4 would be 4/8 = 1 semicircle
a curl of 1 would be 1/8 = 1/8 circle
The value is measured with templates.

The gel body level is determined on 1 m² samples at an angle . The gel bodies are recognizable as small bumps. The level is assessed:
low = <5 gel bodies / m²
medium = 5 - 15 gel bodies / m²
high => 15 gel bodies / m²
Table 2 shows the test results Table 2 Curl Gel body level 80% rel. Moisture after 4 days 14 days B1 - 0.9 - 1.1 unavailable B2 - 0.7 - 0.9 unavailable B3 - 0.6 - 0.9 low B4 - 0.7 - 1.1 unavailable B5 - 0.8 - 1.1 low B6 - 0.8 - 1.1 low V1 - 0.7 - 1.1 high V2 - 0.8 - 1.1 high V3 - 0.6 - 0.9 medium

For the sake of clarity, only one polypropylene copolymer was listed under Example B6. Similar results, which are equivalent in the statement, were also obtained with polypropylene copolymers with 1.5% by weight and 4.0% by weight of ethylene and with a polypropylene block copolymer with 10% by weight of ethylene.

The slight curl (curvature) towards the back of all substrates is deliberate, since this is almost compensated for by the light-sensitive photographic emulsion layers still to be applied.

The results show that by replacing HDPE with polypropylene homopolymer or copolymer in the back coating of photographic supports, the curl behavior can be kept at the same level, but the number of gel bodies can be reduced. This can reduce the reject rate. The recipe can be changed without changing the machine design.

Claims (12)

Photographic support, consisting of a support and synthetic resin layers applied on both sides, characterized in that the synthetic resin layer on the back is wholly or predominantly made from a mixture of low-density polyethylene (LDPE) and polypropylene homopolymer and / or polypropylene copolymer from at least 80% by weight propylene and at most
20 wt .-% of at least one other olefin. Photographic support according to claim 1, characterized in that the support is a photographic base paper. Photographic support according to claims 1 and 2, characterized in that the polypropylene copolymer consists of at least 80% by weight of propylene and at most 20% by weight of ethylene. Photographic support according to claims 1 to 3, characterized in that the content of polypropylene homopolymer and / or copolymer in the synthetic resin layer on the back is 2% by weight to 75% by weight. Photographic support according to claims 1 to 4, characterized in that the polypropylene homopolymer and / or the polypropylene copolymer has a narrow molar mass distribution, preferably <6. Photographic support according to claims 1 to 5, characterized in that the low-density polyethylene is, in whole or in part, a linear low-density polyethylene (LLDPE). Photographic support according to claims 1 to 6, characterized in that the synthetic resin layer on the back contains up to 20% by weight of other polyolefins. Photographic support according to claim 7, characterized in that the other polyolefin is a high density polyethylene (HDPE). Photographic layer support according to Claims 1 to 8, characterized in that the synthetic resin layer on the back contains a total of up to 20% by weight of white pigments, color pigments, optical brighteners, lubricants, release agents, antistatic agents, inorganic and organic fillers and other auxiliaries. Photographic layer support according to claims 1 to 9, characterized in that the layer support contains further functional layers such as primer / adhesive layers, antistatic layers, anti-curl layers, stiffening layers and writable and printable layers. A process for producing a photographic film carrier according to claim 1 to 9, characterized in that - the photographic base paper on the back is pretreated with a corona discharge, - The pre-treated back with the synthetic resin layer for
the back is coated using the melt extrusion process becomes, - The photographic base paper on the front is pretreated with a corona discharge and - The pretreated front is coated with a synthetic resin layer using the melt extrusion process. A process for producing a photographic support according to claim 11, characterized in that further functional layers such as primer / adhesive layers, antistatic layers, anti-curl layers, stiffening layers and layers which can be written on and printed on are applied to the layer support.