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
2003-02-19
2004-09-14
Gorr, Rachel (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S458000, C528S061000, C528S065000, C264S211240
Reexamination Certificate
active
06790916
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a process for the preparation of soft, low-shrinkage molding compositions of thermoplastic polyurethanes, which can be easily released from the mold, have good low-temperature properties, good mechanical properties and a hardness of 45 to 65 Shore A.
Thermoplastic polyurethane elastomers (TPU's) have been known for a long time. They are of industrial importance because of the combination of high-quality mechanical properties with the known advantages of inexpensive, thermoplastic processability. A wide range of variation of mechanical properties can be achieved by using various chemical builder components. An overview of TPU's and their properties and uses is given e.g. in Kunststoffe 68 (1978), 819 or in Kautschuk, Gummi, Kunststoffe 35 (1982), 568.
TPU's are built up from linear polyols, usually polyester or polyether polyols, organic diisocyanates and short-chain diols (chain lengtheners or chain extenders). Catalysts can additionally be added to accelerate the formation reaction. To adjust the properties, the builder components can be varied within relatively wide molar ratios. Molar ratios of polyols to chain lengtheners/extenders of 1:1 to 1:12 have proved appropriate. This usually results in products characterized by a hardness in the range from 80 Shore A to 75 Shore D.
The hardness of a TPU is substantially established by the ratio of hard segment (chain lengthener/extender+isocyanate) to soft segment (polyol+isocyanate). If the amount of hard segment is reduced downwards beyond the limit of 80 Shore A mentioned, the resulting products are tacky products which solidify poorly, exhibit poor releasability from the mold in injection molding processing and exhibit severe shrinkage.
No economically acceptable injection molding cycle times and no adequate dimensional accuracy of the injection-molded components are ensured with such TPU's. In addition, an incipient soft segment crystallization at slightly below room temperature often leads to a significant increase in hardness, and the elastic properties at these low temperatures deteriorate such that the use value of such TPU at low temperatures is reduced.
EP-A 0 134 455 shows that by using plasticizers from specific phthalates and phosphates, TPU's with a hardness of 60 to 80 Shore A are obtained. However, these plasticized TPU, like all plasticized plastics, show disadvantages due to the use of the plasticizers, such as, for example, bleeding of the plasticizer with after-hardening and odor problems. When these are in contact with rigid thermoplastics, stress cracking can occur.
EP-A 1 031 588 describes soft polyurethane molding compositions of low shrinkage in the hardness range from 76 to 84 Shore A by mixing of a 68 Shore A TPU A with an 85 Shore A TPU B. The harder TPU B employed is prepared by a specific prepolymer procedure, the polyol being reacted with the diisocyanate in a molar NCO:OH ratio of 1:2.05 to 1:6.0, so that the shrinkage of the mixture is reduced and a good dimensional accuracy is achieved. This process is of course limited in its effect at very low Shore A values in the range below 75 Shore A.
In DE-A 199 39 112, a previously prepared TPU of 30 to 80 Shore D hardness is degraded in the first part of an extruder with the addition of low molecular weight diols to give the large hard segment blocks; a new soft TPU is then produced in the second part with the addition of isocyanates, polyols and catalysts. These TPU's have good mechanical values and a reduced abrasion. The preparation process described is very involved, and it is therefore very difficult to maintain the TPU properties in a controlled manner. In addition, the ease of release from the mold in injection molding processing is not particularly good.
DE-A 2 842 806 describes the production of TPU in twin-screw kneading machines under special shearing conditions, in which one or two monomer streams are subdivided into at least two portions. TPU's having increased low-temperature notched impact strength and increased rigidity, i.e. Shore hardnesses of higher than 57 Shore D are obtained.
DE-A 4 217 367 describes TPU's in the range from 70 Shore A to 75 Shore D, which are obtained by a multi-stage reaction which is characterized in that, in the first stage, macrodiols are reacted with diisocyanate in a ratio of 1.1:1 to 5.0:1, in the second stage, the remaining diisocyanate is added, and, in the third stage, the reaction with the chain extender takes place. Products are obtained which have improved mould release properties and improved stability under load, while retaining the same hardness and low-temperature properties. TPU's which are softer than 70 Shore A cannot be obtained with the polyesters and polyethers described in the examples using the process described in this reference. If the quantity of the hard segment is reduced to below the limit mentioned of 70 Shore A, products are obtained which, as a result of soft segment crystallization, can only retain their hardness range for short periods of time and subsequently harden to a major degree upon storage or heating.
The object of the present invention was therefore to provide a process with which very soft TPU's in the range from 45 to 65 Shore A can be prepared, which at the same time are easy to release from the mold, show a very low shrinkage, and, in addition, are still highly elastic even at low temperatures.
It has been possible to achieve the object by the process for the production of thermoplastic polyurethane elastomers according to the invention.
SUMMARY OF THE INVENTION
The invention provides a process for the preparation of thermoplastically processable polyurethane elastomers which are easy to release from the mold and have a hardness of 45 to 65 Shore A (as measured in accordance with DIN 53505), a tensile strength of greater than 12 MPa (as measured in accordance with ISO 37), a shrinkage of ≦3.5% (as measured in accordance with DIN 16770, part 3) and a DMA storage E modulus in tension at −10° C. of less than 20 MPa (the measurement of the E modulus is explained in more detail in the examples section) comprising
A) reacting, optionally in the presence of catalysts,
(1) one or more linear hydroxyl-terminated polyols selected from the group consisting of:
a) polyester polyols with number-average molecular weights of 450 to 5,000, which are obtained by reacting a mixture of at least two different polyhydric alcohols with one or more dicarboxylic acids having a maximum of 12 C atoms,
b) mixtures of at least two polyester polyols having different number-average molecular weights of 450 to 5,000,
c) mixtures of at least two polyether polyols having different number-average molecular weights of 450 to 5,000,
d) polyether polyols having number-average molecular weights of 450 to 5,000 and containing at least two different alkylene oxide units, and
e) polyether polyols containing one alkylene oxide unit and having number-average molecular weights of 450 to 1,500, with
(1) one or more organic diisocyanates in a molar NCO/OH ratio of 1.1:1 to 1.9:1, preferably 1.1:1 to 1.7:1 to form an isocyanate-terminated prepolymer,
B) mixing the prepolymer prepared in step A) with preferably the same organic diisocyanate as under step A), and
C) reacting the mixture obtained in step B) with one or more diol chain extenders having molecular weights of 60 to 400, wherein a molar NCO:OH ratio of the components employed in A), B) and C) of from 0.9:1 to 1.1:1 is adjusted and wherein the ratio of the OH groups of the polyol to the OH groups of the chain extender is 0.3:1 to 2.0:1, and particularly preferably 0.4:1 to 1.5:1.
DETAILED DESCRIPTION OF THE INVENTION
Possible organic diisocyanates include, for example, aliphatic, cycloaliphatic, araliphatic, heterocyclic and aromatic diisocyanates, such as are described in, for example, Justus Liebigs Annalen der Chemie, 562, pages 75 to 136.
There may be mentioned specifically by way of example diisocyanates including: aliphatic diiso
Bräuer Wolfgang
Eggeling Eva Brigitte
Heidingsfeld Herbert
Hoppe Hans-Georg
Röhrig Wolfgang
Bayer Aktiengesellschaft
Brown N. Denise
Gil Joseph C.
Gorr Rachel
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