Shape memory hollow body and method of working the same.

0001The present invention relates to a hollow body molded from a thermoplastic polyurethane elastomer having the shape memory function and also to a method of working the same.

0002Polyurethane having a glass transition point as low as about -40°C exhibits a low modulus even at low tempera­tures and hence can be used in the application areas for ordinary natural rubber and synthetic rubber. On the other hand, polyurethane having a glass transition point as high as about 100-110°C exhibits a high modulus and good abrasion resistance even at high temperatures and hence finds use as a raw material for artificial wood. ("Glass transition point" will be referred to as Tg here­inafter.)

0003Meanwhile, the present inventors have proposed a shape memory polyurethane elastomer (in Japanese Patent Laid-open No. 293214/1986). It provides a molding which takes on an as-molded shape and deformed shape when it is heated or cooled to a proper temperature. In other words, the molding takes on a deformed shape when it is deformed at a temperature lower than the molding temperature and then cooled to a temperature below Tg while being kept deformed, so that the deformation is set. The deformed molding restores its original shape when it is heated to a temperature higher than Tg and lower than the molding tem­perature.

0004The reference cited above discloses two kinds of shape memory polyurethane elastomers. One is composed of an isophorone-based isocyanate, a polyol, and an adduct of trimethylol propane with tolylene diisocyanate (as a chain extender). The other is composed of 2,4-tolylene diisocy­anate, a polyol, and 1,4-butanediol (as a chain extender).

0005These polyurethane elastomers contain a large amount of excess NCO groups at the terminals of the polymer mole­cules so that they exhibit rubber elasticity at a tempera­ture higher than the Tg. In addition, the first one has crosslinkage formed by a trifunctional chain extender. Polyurethane elastomers having crosslinkage resemble a thermosetting polymer and hence are limited in the freedom of fabrication. That is, they are very difficult to process by injection molding, extrusion molding, blow molding, or the like.

0006It is an object of the present invention to provide a hollow body molded from a shape memory polyurethane elas­tomer which is thermoplastic and capable of melt molding (such as injection molding and extrusion molding) and exhibits rubber elasticity at temperatures higher than its Tg approximate to room temperature. It is another object of the present invention to provide a method of working said hollow body.

0007The first aspect of the present invention resides in a shape memory hollow body which is molded from a polyure­thane elastomer produced by prepolymer process from a raw material composed of a difunctional diisocyanate, difunc­tional polyol, and difunctional chain extender containing active hydrogen in a molar ratio of 2.00-1.10 : 1.00 : 1.00-0.10, said polyurethane elastomer containing NCO groups and OH groups in almost equal amounts at the termi­nals of the polymer chains and having a glass transition point in the range of -50 to 60°C and a crystallinity of 3 to 50 wt%, said hollow body remembering its as-molded basic shape but taking on a deformed shape which is given when it is deformed at a temperature higher than the glass transition point of the polymer and lower than the molding temperature and then set when it is cooled in the deformed state to a temperature lower than the glass transition point.

0008The second aspect of the present invention resides in a method of working the above-mentioned shape memory hollow body remembering its basic shape, said method com­prising the steps of deforming the hollow body at a tem­perature higher than the glass transition point and lower than the molding temperature and then cooling it in the deformed state to a temperature lower than the glass transition point, thereby setting it in the deformed state, combining the deformed hollow body with another member, and heating the combination to a temperature higher than the glass transition point, thereby causing it to restore its basic shape and achieving firm bonding of the hollow body to the member. <ul id="ul0001" list-style="none"><li>Fig. 1 is a schematic drawing showing the steps of fitting an inner pipe of shape memory polyurethane elasto­mer into a pipe.</li><li>Fig. 2 is a sectional view showing the joining of a shape memory hollow body to a pipe.</li></ul>

0009The conventional shape memory polyurethane elastomer molding depends on rigid allophanate linkages for its rubber elasticity at a temperature higher than its Tg. The allophanate linkages result from the intermolecular crosslinking which takes place when excess terminal NCO groups react with urethane linkages. By contrast, the shape memory polyurethane elastomer in the present inven­tion is a thermoplastic chain polymer which is composed of a difunctional isocyanate, polyol, and chain extender according to a specific formulation but contains no excess terminal NCO groups. It is characterized by a specific crystallinity, a Tg approximate to room temperature, and a specific ratio of moduli measured at temperatures above and below the Tg. This polyurethane elastomer has the partial crystallization instead of the intermolecular crosslinking. Therefore, it is a thermoplastic polymer of chain structure; nevertheless, it exhibits rubber elastic­ity at a temperature higher than its Tg and takes on the as-molded shape and deformed shape at temperatures above and below its Tg. In other words, it has the shape memory function. This thermoplastic polyurethane elastomer can be easily molded into a hollow body by melt molding such as injection molding and extrusion molding.

0010The thermoplastic polyurethane elastomer in the present invention should preferably have a crystallinity in the range of 3 to 50 wt%. With a crystallinity lower than 3 wt%, the polymer will have a low rubber elasticity at a temperature higher than its Tg. Conversely, with a crystallinity higher than 50 wt%, the polymer will have a high rubber elasticity at a temperature higher than its Tg, with the result that the ratio of moduli at tempera­tures 10°C above and below its Tg is smaller.

0011The polyurethane polymer in the present invention is produced from the following raw materials, which are illustrative only and not limitative.

0012The first raw material is a difunctional isocyanate which is represented by the general formula OCN-R-NCO, where R is a group having no or one or two benzene rings. It includes, for example, 2,4-toluene diisocyanate, 4,4′-diphenylmethane diisocyanate, carbodiimide-modified 4,4′-diphenylmethane diisocyanate, and hexamethylene diis­ocyanate.

0013The second raw material is a difunctional polyol which is represented by the general formula OH-R′-OH, where R′ is a group having no or one or two benzene rings. The second raw material may also be a reaction product of said difunctional polyol and a difunctional carboxylic acid or cyclic ether. It includes, for example, polypro­pylene glycol, 1,4-butane glycol adipate, polytetramethy­lene glycol, polyethylene glycol, and an adduct of bisphenol-A with propylene oxide.

0014The third raw material is a difunctional chain extender containing active hydrogen which is represented by the general formula OH-R˝-OH, where R˝ is a (CH₂)_n group or a group having no or one or two benzene rings. It includes, for example, ethylene glycol, 1,4-butane glycol, bis(2-­hydroxyethyl)hydroquinone, an adduct of bisphenol-A with ethylene oxide, and an adduct of bisphenol-A with propy­lene oxide.

0015The thus produced polyurethane elastomer may be rep­resented by the following general formula. HOR˝OCONH(RNHCOOR′OCONH)_nRNHCOOR˝OCONH(RNHCOOR′OCONH)_m­RNHCOOR˝OH where m is 1-16 and n is 0-16.

0016The following illustrates the production of the shape memory polyurethane elastomer. First, a prepolymer is prepared by reacting, in the absence of catalyst, an iso­cyanate component and a polyol component in the ratio shown in Table 1. To the prepolymer is added a chain extender in the ratio shown in Table 1. The resulting mixture is heat-cured to give the shape memory polyure­thane elastomer, which has the basic physical properties as shown in Table 1.

0017In Table 1, Tg represents the glass transition point (°C) and E/E′ represents the ratio of the tensile modulus at a temperature 10°C lower than the Tg to the tensile modulus at a temperature 10°C higher than the Tg. The crystallinity (wt%) was measured by X-ray diffractometry.

0018The hollow body pertaining to the present invention is made of the above-mentioned shape memory polyurethane elastomer. It remembers its basic shape when it is molded. It is given its second shape (deformed shape) when it is deformed at a temperature higher than its Tg and lower than its molding temperature and then cooled in the deformed state to a temperature lower than its Tg. Subsequently, the deformed hollow body is combined with another member, and the combination is heated to a temper­ature higher than its Tg, so that the hollow body restores its basic shape, firmly joining itself to the member. <tables id="tabl0001" num="0001"><img file="EP0367014A2_D0001.tif" /></tables><tables id="tabl0002" num="0002"><img file="EP0367014A2_D0002.tif" /></tables><tables id="tabl0003" num="0003"><img file="EP0367014A2_D0003.tif" /></tables><tables id="tabl0004" num="0004"><img file="EP0367014A2_D0004.tif" /></tables>

EXAMPLES

0019The invention will be described in more detail with reference to the following examples, which are not intended to restrict the scope of the invention.

Example 1

0020In this example, a hollow body (inner pipe) made of the shape memory polyurethane elastomer was fitted into an outer pipe according to the following procedure.

0021First, a polymer having a Tg of 48°C was prepared by prepolymer process according to the formulation of sample No. 40 shown in Table 1. The polymer was made into a cylindrical hollow body <i>1a</i> having an outside diameter of 10.4 cm and a wall thickness of 5 mm by extrusion molding.

0022This cylindrical hollow body <i>1a</i> was heated to about 65°C and then collapsed by pressing. It was cooled to about 40°C in the collapsed state so that it was set deformed. Thus there was obtained a collapsed hollow body <i>1b</i>.

0023The collapsed hollow body <i>1b</i> was inserted into a steel pipe <i>2</i> having an inside diameter of 10 cm. (The resulting combination is indicated by <i>3a</i>.)

0024The collapsed hollow body <i>1b</i> inserted into the steel pipe <i>3a</i> was heated by blowing hot air (about 65°C), so that it restored its original cylindrical shape. Thus there was obtained a composite pipe <i>3b</i> composed of a steel pipe <i>2</i> and a cylindrical hollow body <i>1a</i> of polyurethane elastomer which was firmly bonded to the inside of the steel pipe.

0025This example shows that the use of the shape memory function overcomes the difficulty which was encountered in the past when the inside of a small steel pipe was coated with a polyurethane elastomer.

Example 2

0026In this example, a pipe made of the shape memory polyurethane elastomer was joined to another pipe as shown in Fig. 2.

0027A polymer having a Tg of 40°C was prepared by prepo­lymer process according to the formulation of sample No. 39 shown in Table 1. The polymer was molded into a straight pipe (not shown) having an outside diameter of 5 cm and a wall thickness of 3 mm. This straight pipe remembers its as-molded shape. The end <i>4</i> (3 cm long) of the pipe was heated to about 60°C and then contracted until the outside diameter was 4 cm. The pipe was cooled to about 35°C, with the end contracted, so that the con­tracted shape was set. Thus there was obtained a pipe <i>5</i> as shown in Fig. 2.

0028The end <i>4</i> of the pipe <i>5</i> was inserted into a pipe <i>6</i> having an inside diameter of 4.5 cm. When they are heated to about 60°C, the end of the pipe <i>5</i> restored its original shape, joining itself firmly to the inside of the pipe <i>6</i>. The above-mentioned joining procedure may be applied to joining an elbow made of shape memory polymer to a steel pipe.

0029As the above-mentioned examples show, the thermoplas­tic polyurethane elastomer having the shape memory func­tion can be molded into a hollow body of any shape by melt molding such as injection molding, extrusion molding, and blowing molding.

0030In addition, the shape memory hollow body of the present invention remembers its basic shape, takes on its second shape (e.g., reduced or enlarged diameter, and bending), and restores its basic shape after it has been combined with another member. This property makes it pos­sible to join the hollow body firmly to another member.