Patent Application
20080221247
This invention has as its object a process for gluing together substrates that are made oleophobic and hydrophobic, using an adhesive based on styrene block copolymers. It also has as its object adhesives that make possible the gluing of so-called difficult substrates such as those made oleophobic and hydrophobic.
The gluing of the materials of the packaging industry, in particular paper and cardboard, is a well-known technique. In practice, industrial concerns use a broad range of adhesives, and in particular thermofusible adhesives, very often referred to by the professionals as Hot Melt or HMA, English initials for Hot Melt Adhesives, an abbreviation which will be used below to designate them. These HMA generally consist of the following primary components: a polymer that imparts to the final adhesive its structure, a main “tackifying” adhesion agent, and a plasticizer such as a wax or a mineral oil that brings to the adhesive its thermal and rheological properties.
In the large majority of cases, the choice of adhesive will first be based on the selection of the polymer. The polymer that is used in the HMA consists of, for example, the ethylene-type copolymers, such as vinyl ethylene-acetate (EVA), amorphous poly-alpha-olefins (APAO), polyolefins that are synthesized by metallocene catalysis, polyethylenes of low molecular weight or else amorphous polypropylenes. Also proposed for the polymer are the styrene block copolymers where the latter is combined with a co-monomer such as isoprene or butadiene, hydrogenated forms of these components. The selection will naturally be made on a criterion of structural property but which broadly takes into account the notion of cost of raw materials.
Certain packaging variants, however, pose specific adhesion problems, the “difficult” packages, which is the case in particular of packages (cardboard for cases, kraft paper for bags, etc. . . . ) that are pretreated to impart to them a resistance to liquids (hydrophobic) and to oils and greases (lipophobic). Furthermore, this type of package that should naturally offer all the conventional advantages that are known in terms of protecting materials to be packaged, strength, longevity, appearance or various other functionalities, can be subjected to important variations of temperatures that generally range from −10° C. to +40° C., and even 40° C. to +80° C. Actually, the preparation of food products, for example, as well as their preservation or the conditions in which they are stored, require that the packaging be resistant and continue to ensure its function under extreme temperatures. Thus, the gluing should be effective during all of the packaging operations or the life cycle of the packaging.
The permanent improvement of barrier properties to water, to oils and fatty substances, substrates that are intended for the packaging market, in particular because of the evolution and effectiveness of mass or “size press” treatments (on the back and the front) by increasingly higher-performing fluorinated polymers sometimes makes it difficult to effectively glue this type of substrate with the usual adhesives, in particular when the packaging is subjected to significant temperature variations.
A recent example of this evolution is the use of fluorinated compounds such as the fluorinated polyacrylates in the treatment of substrates that are intended for packaging so as to impart to them oleophobic and hydrophobic barrier properties, namely both at a high level of resistance to water and a high level of resistance to oils and fatty substances.
These fluorinated compounds of various types are used in an aqueous phase and are therefore hydrodilutable. They are either small fluorinated molecules, for example fluorinated phosphate salts, or fluorinated sulfate salts, or fluorinated acrylic copolymers, i.e., copolymers of which at least one monomer comprises a bead(s) being deposited on the substrate(s), the setting speed as well as the machinability are the critical parameters here. The contents being warm, during the closing of the bag, the adhesive is to withstand temperatures on the order of at least 60° C. so as not to reopen.
In the area of the manufacture of cases, it is necessary to note for:
The implementations that are described here are simply examples of the most commonly encountered difficulties.
The adhesive should be able to be used for any assembly of which at least one of the substrates has undergone a hydrophobic and oleophobic treatment.
The adhesive should therefore have very specific and sometimes contradictory characteristics, short setting time, good wetting power, very good thermal behavior of the adhesion after temperature cycles located between −10° C. and +60° C., and even between −40° C. and +80° C., as well as a low viscosity because of the fluorinated treatment.
The process according to the invention is used in the sense of resolving this problem, in particular by a selection of the polymer and its structure.
The invention has as its object a process for gluing together so-called difficult substrates that are intended for the manufacture of packages, which consists in applying on at least one of the substrates an adhesive that comprises:
The adhesive of the process of the invention optionally can comprise various additives such as antioxidants . . . .
Component a) according to the process of the invention (the copolymer), has a diblock, triblock or multiblock, linear, radial or star-shaped structure, whereby the intermediate block consists of at least one of the comonomers listed above and can undergo a hydrogenation phase. The block copolymer or the mixture of block copolymers comprises a mass percent of the styrene phase in the polymer that is between 10 and 40% and preferably between 20 and 35%, a mass percent of diblock structures in the variable polymer generally encompassed between 0 and 50%, a flow index (MFI=Melt Flow Index) that is measured according to condition No. 10 of the NFT 51-016 standard of between 2 and 70 g10 min.
Component a) is preferably a styrene-ethylene butylene-styrene block copolymer (designated SEBS) or any other close structure. It is possible to mix this component a) with other polymers such as the copolymers of ethylene, polyolefins, polymers that are obtained by a metallocene catalysis path. The content of the adhesive in component a) in general between 5 and 50% by mass is preferably between 15 and 30%.
For the most part, component b) is selected from among the resins that are conventionally used in the thermofusible adhesives, such as:
Component c) consists of resin or a mixture of resins that are conventionally used, such as the polymers that are obtained from aromatic fractions or else poly-alpha-methylstyrene. It is preferably selected from among the resins that are obtained from the polymerization of alpha-methylstyrene, for example having a softening point that is measured according to the EN 1238 standard encompassed between 80 and 160° C. The content of component c) is between 0 and 20% by mass and preferably between 5 and 15%.
Component d) is selected for the most part from among the so-called microcrystalline waxes that are conventionally used in the thermofusible adhesives, having a melting point (ASTM D127 method) of between 70 and 100° C., preferably between 80 and 95° C. In a minority amount, component d) can contain synthesis waxes such as polyolefins with short chains that are rather linear, obtained by polymerization according to standard processes (Ziegler Natta; Fischer Tropsch), whose melting point that is measured according to the standard (ASTM D127 method) is high, generally between 80 and 150° C. and preferably between 90 and 120° C. The content of component d) is between 5 and 25%, and preferably between 10 and 17% by mass.
Polymers of another nature can be present in a minority way in the adhesive, for example the ethylene-vinyl acetate copolymers (EVA), the polyolefins of the processes or of various natures, and the other styrene block copolymers.
The various additives will be, for example, the antioxidants that are conventionally used in the thermofusible adhesives or in the industry for transformation of thermoplastic compounds, such as hindered phenolic derivatives, phosphites or mixtures thereof.
The viscosity of the thermofusible adhesive of the process of the invention will be between 400 and 3000 mPa·s at 170° C. and preferably between 700 and 1400. Its softening point that is measured according to the so-called ball-ring method that is known to one skilled in the art will be encompassed between 75 and 120° C.
The adhesive according to the process of the invention is obtained by mixing a), b), c), d) and e) as well as the possible additives, by any suitable means, for example by a simple mixing at a temperature of between 150 and 170° C.
The adhesive can be easily characterized by chemical analysis according to the standard processes for deformulation and identification of different fractions, in particular by infra-red spectrometry, nuclear magnetic resonance 1H and 13C, elementary micro-analysis, gel permeation chromatography or high-performance chromatography or else by differential calorimetry (DSC or Differential Scanning Calorimetry).
On a practical plane, the adhesives of the process of the invention are applied according to the standard methods that are used in the area of thermofusible adhesives, on the packaging chain or else on the edge of the latter. Starting from a boiler tank, via heated and heat-insulated pipes, by extrusion nozzles: controlled deposition of one or more beads on a first substrate and mating with the second substrate, with optional pressing of the adhesive joint thus produced. The low-viscosity HMA can be applied by any other possible means such as, for example, a lipped nozzle, multi-line nozzles, disk, imprint or else by the Sift Proof process developed by the Nordson Company. The temperature of the adhesive at the time of application is conventionally between 150 and 180° C.
This type of process is conducted according to certain specific parameters. For example, the machine open time is the time that passes between the application of the adhesive on the first substrate and mating with the second substrate. The machine pressing time is the time immediately after gluing during which the two glued substrates are kept in contact with a force that is at least equal to the force of reopening the packaging without adhesive.
The adhesive is applied on thin substrates, i.e., of a thickness that is generally encompassed between 0.05 and about 2 mm, whereby these substrates can be part of a thicker complex structure. This substrate often can be based on paper or cardboard, such as virgin or recycled kraft paper, having a low density or on the contrary a compact thin substrate, optionally treated by fluorinated compounds or on the surface by acrylic varnish or UV-reticulated varnish, or of specific coatings, optionally with non-coated zones, reserves, to make possible the deposition and the hooking of the adhesive on the substrate.
The nature of the various substrates used is selected from among identical or different or composite materials, most of the time based on paper or cardboard, metal, for example, aluminum, or plastics, such as polyethylene, polypropylene, terephthalate polyethylene, polystyrene, . . . .
The process of the invention is illustrated by 6 examples in which were compared, under conditions representative of the use, the performance levels of various HMA formulas of the prior art and HMA according to the invention.
To evaluate the performance levels of the HMA in the process according to the invention, various tests are carried out: the determinations of viscosity, open time, setting time, heat behavior and temperature behavior (−10° C. to +60° C.).
The substrates that are used for our tests are FORAPERLE® 325 treated papers or cardboards with a concentration of about 3%.
By measuring the oleophobia on these cellulose fibrous substrates, according to the standardized method of the TAPPI 559 test kit, values of between 8 and 12, and even greater than 12, are found. In addition, the typical characteristics for the kraft paper are as follows:
The raw materials that are used in the examples are as follows:
The examples including the following comparison examples whose results are combined in the Table, pages 12 and 13, will make the invention better understood.
Examples 1 and 2 target first-generation adhesives of the prior art based on EVA (Example 1) and APAO (Example 2); Examples 3 and 4 are comparison examples that use SEBS bases described in the prior art and Examples 5 and 6 of the examples based on SEBS according to the process of the invention.
Based on these examples, it is noted that:
aComparison;
bAccording to the invention;
cResults expressed in % fiber extraction;
| Number | Date | Country | Kind |
|---|---|---|---|
| 01/12654 | Oct 2001 | FR | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10815914 | Apr 2004 | US |
| Child | 12124036 | US | |
| Parent | PCT/FR02/03335 | Oct 2002 | US |
| Child | 10815914 | US |
