Tailored rheology hotmelt spray composition.

The invention relates to an easily sprayable hot melt composition based on a largely amorphous polyalphaolefin for hot melt adhesive applications.

In the field of hotmelt adhesives and hotmelts, in addition to the proven application methods such. As roller, caterpillar, slot nozzle application, etc., spray technology has also found its way in recent years. A basic distinction must be made here between the so-called "atomizing" of the melt and the so-called "spin spraying", which is sometimes also referred to as "controlled fiberization". The first-mentioned process involves atomizing the melt into fine droplets of melt, while in "spin spraying" a melt thread leaves the spray nozzle in a twist-like manner and, without tearing off, is deposited in a spiral path on the substrate to be coated. Spray technology generally allows contactless application and the coating of uneven, irregular surfaces. It is also suitable for applications where, on the one hand, flat gluing is desired, on the other hand, it is required that the glued layer is permeable to air or moisture. The fact that there is no full-surface application means that material can be saved. Another advantage is shown when coating temperature-sensitive substrates. Due to the contactless application and due to the very low heat content of the incoming melt or of the melt thread, the substrate is stressed or damaged far less than with conventional application technologies. The hotmelt spray technology is environmentally friendly and covers a total of many applications that were previously reserved for the sprayed solvent-based adhesives.

With sprayable hot melts it is state of the art that formulations based on thermoplastic rubber and also formulations based on ethylene vinyl acetate are easy to spray. However, formulations based on thermoplastic rubber are expensive and also contain relatively high amounts of oil, which is undesirable in some cases. Formulations based on ethylene vinyl acetate often have an "open time" which is too short, as a result of which their processing time is greatly restricted in time. For example, this can mean that, in the event of delays in the production process, such a formulation no longer has any adhesive strength before the actual application.

Largely amorphous polyalphaolefins do not show these disadvantages and are generally also considered sprayable. On closer inspection, however, it turns out that due to their rheological properties they do not lead to a uniform spray pattern (ie to an uneven application) and that there are often faults in the spraying process, e.g. B. formation of drops on the substrate ("blobs"). The latter can be used with temperature sensitive substrates such. B. LDPE foils lead to damage because the adhesive drop arriving on the substrate has too high a heat content. Regarding the rheological properties of largely amorphous polyalphaolefins, it must be said that they have a pronounced structural viscosity and thus no Newtonian flow behavior. This means that their viscosity depends very much on the shear rate, which is relatively high in the area of ​​the spray nozzle, but decreases shortly after leaving the nozzle. For a uniform spray pattern - if the spray pressure is kept constant - a viscosity value that is largely independent of the shear rate is required at the corresponding application temperature. Diagram 1 shows viscosity curves of a typical amorphous polyalphaolefin (viscosity at 190 ° C approx. 8,000 mPas) as a function of the temperature and under various shear stresses.

In the case of polyalphaolefins with a relatively low melt viscosity (less than 1,000 mPas, determined at 190 ° C), such serious differences in the viscosity curve cannot be observed (diagram 2), but these products either have too low a molecular weight (i.e. low cohesion) or too high crystallinity (ie poor adhesion), so that an acceptable adhesive bond cannot usually be achieved.

The spray pattern can in principle also be improved if the spray temperature is selected so that it is at a suitable distance above the so-called "critical spray temperature" (the "critical spray temperature" is the temperature above which the shear rate no longer has any significant effect on the viscosity; it is well above 200 ° C for commercially available amorphous polyalphaolefins). The disadvantage, however, is that temperature-sensitive substrates cannot be sprayed and that the material to be sprayed is subjected to a much greater thermal load in the melt container, which can lead to degradation reactions, discoloration, odor formation, etc. An increase in the spray pressure, as far as the spray pattern is concerned, would also be advantageous, since one would move on a flatter viscosity / temperature curve due to the higher shear rate, ie the temperature dependence of the viscosity would be less. However, high spray pressures put a strain on certain substrates (e.g. nonwovens) and also lead to increased dust and fiber swirling (e.g. when fixing nonwovens), so that fully automated production processes can cause faults in light barriers.

Surprisingly, it was found that in the case of largely amorphous polyalphaolefins, the desired rheology can be set in a targeted manner via the molecular weights, so that corresponding products do not show these disadvantages. The polyalphaolefins according to the invention have the following features:<ul id="ul0001" list-style="dash"><li>their softening point, measured according to the ring and ball method according to DIN 52 011, is between 70 and 130 ° C;</li><li>their melt viscosity at 190 ° C is between 1,000 and 20,000 mPas;</li><li>their density is less than 0.90 g / cm³;</li><li>the needle penetration 100/25/5, measured in accordance with DIN 52 010, is between 8 and 40 0.1 mm, (weight of the needle: 100 g; temperature 25 ° C; duration 5 s);</li><li>the molecular weight of the largely amorphous polyalphaolefin determined by gel permeation chromatography is a maximum of 80,000 in weight average (Mw) and at least 4,000 in number average (Mn), the difference formed from the weight average and the number average molecular weight not exceeding six times the number average. In other words, this means that the so-called "non-uniformity" may not exceed 6. Such polyalphaolefins are excellently sprayable at low temperatures (below 200 ° C.) and low spray pressures (between 0.5 and 4 bar) and lead to a uniform spray pattern.</li></ul>

Preferred polyolefins are either completely amorphous or have only a low crystallinity. In general, a degree of crystallinity of 25%, determined by X-ray diffraction, should not be exceeded.

Suitable products can be produced, for example, by radical degradation of marketable, largely amorphous polyalphaolefins with softening points between 80 and 140 ° C and viscosities between 5,000 and 100,000 mPas at 190 ° C. This manufacturing method is the subject of German patent application P 40 00 695.6 on the same day. However, they can also be produced by any other method, provided that they subsequently have the features as claimed.

Diagram 3 shows the viscosity curve of such a product according to the invention as a function of the shear rate and the temperature. The "critical spray temperature" is below 180 ° C.

Suitable, largely amorphous polyalphaolefins can carry functional groups to improve the adhesion to conventional substrates. These functional groups can be introduced either by copolymerization with small amounts of functional monomers or preferably by radical reaction of commercially available polyalphaolefins with such functional monomers. Suitable monomers include maleic anhydride, fumaric acid, acrylic and methacrylic acid, itaconic acid, aconitic acid and their derivatives such as. B. esters or amides and vinyltrimethoxysilane (VTMO) and 3-methacryloxypropyltrimethoxysilane (MEMO; H₂C = C (CH₃) COO (CH₂) ₃Si (OCH₃) ₃). They are usually used in amounts of 0.1 to 10% by weight, preferably 0.5 to 5% by weight, based on the polyalphaolefin. Common radical starters such. B. Dicumyl peroxide or 2,2'-azo-bis (2-acetoxypropane) is used in amounts of 0.05 to 3% by weight, preferably 0.1 to 2% by weight. The grafting reaction then takes place at elevated temperatures, usually between 100 and 300 ° C. In this way, polymers with high cohesion and improved adhesion to certain substrates such as metal, plastic or glass surfaces are obtained.

In a preferred embodiment, the largely amorphous polyalphaolefin is a binary or ternary copolymer of olefins having 2 to 10 carbon atoms. In the preferred range, this copolymer has the following monomer composition: 3 to 75% by weight of an alpha olefin with 4 to 10 carbon atoms, 25 to 95 wt .-% propene and 0 to 20 wt% ethene.

In addition, as already mentioned, functional monomers can also be bound.

In a particularly preferred embodiment, butene-1 is used as the alpha-olefin having 4 to 10 carbon atoms.

Mixtures of various polyalphaolefins according to the invention can of course also be used.

The sprayable hot-melt composition can contain the additives customary in the hot-melt sector, the proportion of which, however, should not exceed 30% by weight and is preferably 5 to 20% by weight. In this context, waxes (e.g. microcrystalline waxes, synthetic waxes of the Fischer-Tropsch or polyolefin type) and / or resins (e.g. modified hydrocarbon terpene resins, polyterpene resins, aliphatic hydrocarbon resins, hydrogenated rosin esters) should primarily be considered. In the case of waxes, micro waxes in particular have proven to be favorable. The use of paraffinic and aromatic oils is also possible. Of course, the hot-melt composition according to the invention can also contain the usual additives for plastics, such as. B. heat and light stabilizers, optical brighteners, antistatic agents, lubricants and antiblocking agents, nucleating agents, fillers and dyes, pigments and flame retardants.

The improvement in sprayability naturally relates both to the "atomizing" mentioned above and to "spin spraying". As with the atomization described in the following examples, a uniform spray pattern produced by spin spraying at the respective application temperature also requires a viscosity of the mass to be sprayed which is largely independent of the shear rate. If this were not the case, it would not be possible, for example, to achieve a constant spray width in the case of intermittent spray processes. Both at the beginning and at the end of the respective spray cycle, the viscosity would be increased due to the lower shear rate and the spray pattern would consequently change.

Examples

The spray tests were carried out using the CL 200 device from Meltex Verbindungs-Technik GmbH, D-2120 Lüneburg. The Meltex EP 26 SD model was used as the spray head. This is a so-called double air nozzle. The melt conveyed via a gear pump (quantity can be varied) leaves the nozzle through an opening of approximately 1 mm in diameter. An annular slot nozzle is concentrically attached around the outlet hole, through which hot air ("internal air") is pressed. Guides in front of them give the hot air a swirl. In addition to this ring nozzle there are 4 further air outlet holes ("outside air"). On the one hand, this causes the spray cone to be pushed together to form an elliptical base area, and on the other hand, a more precise side boundary.

The ratio of indoor to outdoor air volume must be optimized for each spray attempt - depending on the substance to be sprayed and the other device parameters - with regard to a uniform spray pattern. This inevitably results in differences in the spray width. For a given viscosity, a product that is easy to spray provides a uniform and at the same time wide spray pattern.

In the tests, apart from the spray pressure and the ratio of outside and inside air to be optimized depending on the test, all other device parameters including the application quantity were kept constant:<tables id="tabl0001" num="0001"><img file="EP0442045A2_D0001.tif" /></tables>

Were assessed visually<ul id="ul0002" list-style="dash"><li>Uniformity of the spray pattern</li><li>Spray edge limitation</li><li>Thread count</li><li>No interference during the spraying process (ie no blob formation on the substrate)</li></ul> A grading scale from 1 to 6 (1 = very good, 6 = bad) was created for this. The spray width was measured, and the grading on the grading scale was carried out as follows:<tables id="tabl0002" num="0002"><img file="EP0442045A2_D0002.tif" /></tables>

The following products and formulations were used:<ul id="ul0003" list-style="none"><li>A: Atactic polypropylene not according to the invention<img file="EP0442045A2_D0003.tif" /></li><li>B: Amorphous ethene / propene / butene-1 terpolymer not according to the invention<img file="EP0442045A2_D0004.tif" /></li><li>C: Non-inventive mixture of a propene / butene-1 copolymer (viscosity 2,500 mPas) and an ethene / propene / butene-1 terpolymer (viscosity 21,500 mPas), ratio 40:60</li></ul><tables id="tabl0003" num="0003"><img file="EP0442045A2_D0005.tif" /></tables><ul id="ul0004" list-style="none"><li>1. Ethene / propene / butene-1 terpolymer according to the invention<img file="EP0442045A2_D0006.tif" /></li><li>2nd Ethene / propene / butene-1 terpolymer according to the invention<img file="EP0442045A2_D0007.tif" /></li><li>3rd Mixture according to the invention consisting of 80 parts by weight of the terpolymer of 2 and 20 parts by weight of plastic micro wax (Lunaflex^(R) MB)<img file="EP0442045A2_D0008.tif" /></li><li>4th Mixture according to the invention consisting of 80 parts by weight 2 and 10 parts by weight of plastic micro wax (Lunaflex^(R) MB) and 10 parts by weight of hydrocarbon resin (Escorez^(R) 1102)</li></ul><tables id="tabl0004" num="0004"><img file="EP0442045A2_D0009.tif" /></tables>