Thermoplastic polyurethane

Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...

Reexamination Certificate

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C525S440030

Reexamination Certificate

active

06538075

ABSTRACT:

The invention relates to thermoplastic polyurethanes with a Shore A hardness of from 45 to 80, preferably from 45 to 71, a tensile strength to DIN 53504 of above 15 MPa, preferably from 18 to 40 MPa, a tear-propagation resistance to DIN 53515 of above 30 N/mm, preferably from 35 to 60 N/mm, and abrasion to DIN 53516 of less than 250 mm
3
, preferably from 150 to 40 mm
3
. The invention further relates to a process for preparing thermoplastic polyurethanes with a Shore A hardness of from 45 to 80, and also to the thermoplastic polyurethanes which can be prepared by this process.
Thermoplastic polyurethanes, also referred to below as TPUs, and also processes for their preparation, are well known and have been described many times. These TPUs are semicrystalline materials and belong to the class of thermoplastic elastomers. They feature, inter alia, good strength, abrasion, tear-propagation resistance and chemicals resistance, and can be prepared at almost any hardness desired by using appropriate raw material formulations. They also have advantages in terms of cost-effective preparation, for example using the belt process or the extrusion process, which can be carried out continuously, and in terms of easy thermoplastic processing.
Thermoplastically processable polyurethane elastomers based on aromatic diisocyanates are usually prepared by one of two continuous processes, the belt process and the extrusion process. Hardness and crystallinity, and also processing performance, may readily be influenced and varied here via the molar ratio of chain extender/polyol. However, if a hardness in the range Shore A<80 is desired the reaction rate falls, primarily due to the small amounts of chain extenders, and the products become tacky, are defective in terms of crystallization and are difficult or impossible to process by injection molding or extrusion.
One method of obtaining relatively soft TPUs with hardness of up to Shore A 60 is to start with a TPU having a hardness of Shore A 85 to 90 and add from 20 to 40% of plasticizer. These plasticized TPUs crystallize well and process well, but, like all plasticized plastics, have certain disadvantages caused by using the plasticizer, for example bleed-out of plasticizer and plasticizer odor.
Other attempts to prepare relatively soft TPUs have consisted in modifying the structure of the TPU polymer chain, in particular the size of what is known as the hard-phase blocks, by forming prepolymers from long-chain polyols and an excess of diisocyanate and then reacting with a low-molecular-weight diol chain extender.
In EP 571 828, for example, long-chain polyol is reacted with an excess of MDI to give a prepolymer, which is then reacted with 1,4-butanediol chain extender to give a high-molecular-weight TPU with a Shore A hardness of from 85 to 98. DE-A 196 259 87 gives further modification of this principle.
In DE-A 28 54 409 thermoplastic polymers previously produced, including TPUs, for example, are metered into barrel section 1 of an extruder, while linear polyols, chain extenders and aromatic diisocyanates are fed into barrel section 2 or into other barrel sections. This technical teaching does not permit a reaction or even a controlled cleavage of the TPU with chain extenders for controlled preparation of hard phases which crystallize well, since the compounds reactive to isocyanates immediately react with the isocyanate which is introduced at the same time. A process of this type leads in practice to nothing more than a mixture of a previously prepared high-molecular-weight TPU with a high-molecular-weight soft phase made from polyol, MDI and chain extender.
It is an object of the present invention, therefore, to develop thermoplastic polyurethanes with a Shore A hardness of from 45 to 80 which, without the use of plasticizers, have improved mechanical properties, in particular improved tensile strength and tear-propagation strength and reduced abrasion. The TPUs should also have excellent suitability for processing by injection molding or extrusion. Processes for preparing thermoplastic polyurethanes of this type with a Shore A hardness of from 45 to 80 were also to be developed.
We have found that this object is achieved by the thermoplastic polyurethanes described at the outset and by the processes set out below.
The novel thermoplastic polyurethanes are preferably based on reacting thermoplastic polyurethanes with a Shore D hardness of from 30 to 80, preferably from 40 to 80, particularly preferably from 40 to 70, with a diol (c) which has a molar mass of from 62 to 700 g/mol, preferably from 62 to 250 g/mol, and with an isocyanate (a), and also, if desired, catalysts (d) and/or compounds (b) having a molar mass of from 701 to 8000 g/mol and reactive to isocyanates.
The novel TPU products particularly preferably derive from reacting a mixture comprising 15 to 50% by weight of the hard thermoplastic polyurethanes, preferably with a D hardness of from 30 to 80, from 0.15 to 3.0% by weight of (c) and from 84.85 to 47% by weight in total of (a) and, if desired, (b), where the percentages given are based on the total weight of the mixture.
The novel process for preparing the thermoplastic polyurethanes with a Shore A hardness of from 45 to 80 may comprise reacting, in a first step (i), a thermoplastic polyurethane with a diol (c) which has a molar mass of from 62 to 700 g/mol, preferably from 62 to 250 g/mol, and then, in a further reaction step (ii), reacting the reaction product from (i) with an isocyanate (a) and also, if desired, with (b) compounds having a molar mass of from 701 to 8000 g/mol and reactive to isocyanates, and with (c) diols having a molar mass of from 62 to 700 g/mol, preferably from 62 to 250 g/mol, and with (d) catalysts and/or with (e) auxiliaries and/or additives, preferably with (b) compounds having a molar mass of from 701 to 8000 g/mol and reactive to isocyanates. The step (i) provides the hard phases for the end product via the TPU used in step (i), while in step (ii), in particular through the preferred use of component (b), the build-up of the soft phases takes place.
The technical teaching according to the invention is that TPU with a Shore D hardness of from 30 to 80 and with a well developed hard-phase structure which crystallizes well is first melted in a reaction extruder and degraded using a low-molecular-weight diol to give a prepolymer with hydroxyl end groups. The original hard-phase structure which crystallizes well is substantially retained and can then be utilized to obtain relatively soft TPUs with the advantageous properties of the hard TPU, such as a granular structure which is free-flowing and permits easy feeding, good mold-release performance and high injection-molding speed.
The starting material, i.e. the thermoplastic polyurethane, which is reacted with (c) in step (i), preferably in the molten state and particularly preferably at from 200 to 250° C., and preferably for 0.1 to 4 min, particularly preferably from 0.1 to 1 min, may be a well known thermoplastic polyurethane, for example in pelletized form. These TPUs preferably have a Shore D hardness of from 30 to 80, since TPUs of this hardness have a well developed hard-phase structure which crystallizes well. The TPU to be used in this step may be based on the starting components (a), (b) and, if desired, (c), (d) and/or (e) described below and may have been produced by well known processes, such as the one-shot process or the prepolymer process, on belt systems or in reaction extruders.
The weight ratio of TPU to component (c) in step (i) is usually from 100:0.5 to 100:10, preferably from 100:1 to 100:5.
The reaction of the TPU with the short-chain diol (c) in reaction step (i) is preferably carried out in the presence of usual catalysts (d), for example those described below. The use of metal-based catalysts is preferred for this reaction. The reaction in step (i) is preferably conducted in the presence of from 0.1 to 2% by weight of catalyst, based on the weight of component (c). The reaction in the presence of catalysts of this typ

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