Process for the production of polyurethane particles

Details Process for the production of polyurethane particles Process for the production of polyurethane particles

2001-08-10

2003-04-15

Gorr, Rachel (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

From reactant having at least one -n=c=x group as well as...

C525S440030, C264S014000

Reexamination Certificate

active

06548619

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for the particulate preparation of heat-activatable polyurethanes from the solution or the melt. The particles produced according to the invention are suitable for bonding different substrates.
2. Description of the Prior Art
Heat-activatable polyurethanes are worked up from solutions or melts thereof by dissolving a compressible auxiliary agent under pressure into the initial batch of solution or melt and by means of an expansion device spraying the solution obtained, such that particles result which are finally separated from the stream of expanded auxiliary agent and optionally of solvent. Such particles are suitable as adhesives or for the manufacture of adhesives.
Heat-activatable polyurethanes are segmented polyurethanes based on crystallizing oligomeric dihydroxy compounds having a molecular weight of basically between 500 and 5000 g/mol, preferably polyesters, as an option supplemented by amorphous oligomeric dihydroxy compounds, furthermore aromatic or aliphatic diisocyanates, as an option low molecular weight difunctional chain extenders, and, also as an option, further additives such as light stabilisers, antioxidants, powdering agents as well as polyfunctional cross-linking molecules, preferably isocyanates in free or blocked form.
Such compounds are distinguished in that their so-called soft segments decrystallise at a temperature of, for example, 50° C. which is still comparatively low. This substance-dependent temperature is termed hereinbelow the “crystallite melting point”, and it can be determined by DSC, for example.
The use of heat-activatable polyurethanes as adhesives for bonding the most varied materials to themselves and to other materials is known from, for example, DE-A-1 256 822 and DE-A-1 930 336.
Heat activation can in practice be achieved by brief irradiation with infrared light or by a short residence time in a hot air oven or heating tunnel. In the heat-activated state the adhesive film is tacky and can be joined. The flow behaviour of these polymers is, on the other hand, determined by the strong intermolecular interactions of their urethane groups, such that although still in the activated state, the adhesive film in the bonded joint builds up a very high immediate strength and takes on the resilience of the adherends. Moreover a long-term service temperature of the adhered bond, which is markedly above the crystallite melting point is achieved thereby.
The recrystallisation of the soft segments after thermal activation takes a certain amount of time which, depending on the chemical composition of the polyurethane and the ambient temperature, may last from minutes to hours. It can, for example, be tracked by taking repeated measurements of the Shore A hardness of cooling polymer films. The delayed recrystallisation affords a specific temporal window within which the film adhesive can be readily joined, that is to say with slight pressure and within a short contact time. This is naturally also dependent on the joining pressure and joining time and is in practice generally between a few seconds and some minutes. This period is frequently termed the “hot tack life” (Festel et al., Adhäsion, No. 5, 1997, 16).
As a result of these specific properties heat-activatable polyurethanes meet the requirements of modern joining technology, that is to say they provide high immediate strength with simultaneously a long processing time after heat-activation.
It is known that adhesives based on heat-activatable polyurethanes may be used not only as solutions in organic solvents or as aqueous dispersions (H. W. Lucas et al., Adhesives Age No. 2, 1997, 18), but also in solvent-free or carrier medium-free manner, in the form of film adhesives (H. J. Studt, Coating No. 2, 1993, 34), adhesive nets (J. Hürten et al., Adhäsion No. 3, 1997, 34) or adhesive fleeces (EP-A-0 628 650), as well as in the form of adhesive powders or adhesive pastes (H. Simon, Adhesives Age No. 8, 1998, 28). Powdered adhesives are becoming increasingly important in modern joining technology, for instance for the bonding of textiles. The different flat textile structures based, for instance, on cotton, cotton mixed textiles, wool, wool mixed textiles, polyester and polyamide textiles as well as polyolefins, might in particular be named as substrates which are considered in this context. Here, particle sizes below 600 &mgr;m are suitable for simple scatter applications, however particle sizes below 200 &mgr;m and in particular below 100 &mgr;m, which are suitable for the paste dot or double dot process, for example, are preferred.
The heat-activatable polyurethanes are prepared by polyaddition of the aforementioned adducts in a concentrated solution (stirred reactor technique) or melt (reaction screw technique, mixing head technique). (U. Barth, Plastverarbeiter 40 (1989) No. 1). Since in a solution process in accordance with current practice the process solvent is first separated by vaporisation, the high-viscosity polyurethane melt constitutes an intermediate which is independent of the process and must be converted by a suitable process step into a tack-free, processable product. Under process conditions (that is to say at temperatures of from 130 to 180° C. and shear rates of from 10 to 300 s
−1
) the melt viscosities of such polyurethanes are generally above 1,000 Pa·s (measured in a high-pressure capillary viscometer with 30/2 mm nozzle geometry, (model Rheograph 2002, from Göttfert, DE).
Prior art in the stirred reactor and reaction screw processes is to granulate the polymer melt by way of an extruder into a circulating turbulent cold water stream, whereby the length of the closed circular pipeline is calculated such that on its single passage each granule particle has a residence time in the region of a few minutes, in order to become tack-free as a result of the advancing soft segment crystallisation. If, on the other hand, the granules reach the separator and the downstream apparatus prematurely, there is a risk of agglomeration and blockage. It is essential to remember that for this it is not the surface temperature of the granules suspended in the water stream which constitutes the limiting factor, but the delayed recrystallisation caused by the chemical composition.
In addition to the considerable capital cost and operating and maintenance costs of industrial-scale screw machines and an infrastructure of water circuits, separators, dryers, conveying apparatus and the like, a further disadvantage is the high stressing of the product due to heat and shear forces during the extrusion phase and also, in the case of the solution process, during the evaporation phase, which, in particular in conjunction with subsequent contact between the melt and the granulating water is always associated with the risk of undesirable chain degradation and thermooxidative ageing. Moreover, even when the so-called microgranulation technique is used, the particle sizes cannot be reduced below approximately 1 mm.
Processes in which the product, which is cast by way of a mixing head onto a belt (in continuous operation) or into slabs or blocks (in batch operation) and heat-treated, is peeled from the belt or, manually, from the moulds after cooling and then ground to the desired particle size, are an alternative. In this case the material being ground must be prevented by suitable cooling measures from heating beyond the crystallite melting point as a result of the grinding energy input, which could result in blocking. When this is scaled up into the range which is relevant to industry, however, the costs of such processes, which rise in linear fashion with the installed capacity, very soon exceed the comparable outlay on a stirred reactor or reaction screw process. There are moreover reservations concerning the occupational health aspects of the mixing head process in batch operation.
Powder-like particle sizes can be obtained with the described processes only by a special cryogenic grinding technique w

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