CA1081895A

CA1081895A - Thermally stable polyurethane elastomers produced from poly(oxypropylene)-poly(oxyethylene) glycols of high oxyethylene group content

Info

Publication number: CA1081895A
Application number: CA245,028A
Authority: CA
Inventors: Francis X. O'shea
Original assignee: Uniroyal Inc
Current assignee: Uniroyal Inc
Priority date: 1975-09-11
Filing date: 1976-02-04
Publication date: 1980-07-15
Also published as: AU502071B2; FR2323716B1; SE7610054L; JPS5233998A; DE2640851A1; FR2323716A1; AU1767876A; US3983094A; GB1548979A

Abstract

ABSTRACT OF THE DISCLOSURE

Flexible automobile exterior body parts are molded from a polyurethane elastomer prepared from a reaction mixture comprising:
a) a poly(oxypropylene)-poly(oxyethylene) glycol of molecular weight of from about 1750 to about 4000 containing 15 to 50% by weight oxyethylene groups, b) methylenebis(4-phenylisocyanate);
c) 1,4-butanediol.
The invention also relates to this polyurethane elastomer.

Description

108~895 ,, . --1 .

This invention relates to a polyurethane elasto-mer and to shaped artlcles prepared therefrom.
Flexible exterior body parts for automobiles, including parts associated wlth energy-absorbing bumpe~
systems such as sight shields, fender extensions and full fascia ~ront and rear ends, require a material uith a particular set o~ propertles~ The material must be capable of ~lexing under impact and then returning t~ -its original shape. Therefore, it must be elastomeric in nature. It must have strength as typ~fied by high ten-sile strength and high tear strength.
In addition, there are two more stringent require-ments. It must be capable of withstanding dynamic ~m-pact at -20F. and it must be resistant to distortion at 250F. The latter requirement is imposed by typical con-~; dltions under which painted pieces are dried.
One class o~ materials which has been used for thls purpose is~polyurethane elastomers. Polyurethane ~ ;
ela9tomers are "blockr type polymers resulting ~rom the ~ 20 ~ ~reaction o~ a polymeric diol of molecular weight o~ fr~m -p ~ about 500 to 5000 with a dilsocyanate and low molecular ~ ~ we~ght difunctional compound commonly referred to as the ;~ ~ "chaln extendor". The chain extender has a molecular weight below 500 and generally below 300.
~ me polymeric dlol ls recognized as the "soft"
segment o~ the elastomer, con~erring elasticity and soft-ness to the polymer. Typically, thiæ component has a molecular weight of about 1000 to 2000 and may be a poly-; (alk~lene ether? glycol such as poly(tetramethylene ether) glycol or poly(oxypropylene) glycol, a polyester diol, a ~ ~ , ~ 1 .... . . ., .... .,. ,. . , ....... .. , . , ,, : . .

~ 1081895

2-polycaprolactone diol or polybutadiene dlol.
The combination of the diisocyanate and the chaln extender comprises the "hard" segment of the elastomer, contributing rigidity and strength. Typical diisocyanates include 2,4-tolylene diisocyanate and methylene~is(4- -phenylisocyanate). The ~hain extenders are ~ypically diamines or diols. Typical diols whlch may be used are listed, for example, in U.S. Patents 3,233,025 (col. 4, . lines 20-26), 3,620,905 (col. 2, lines 53-59) and 3,718,622 ~col. 2, lines 10-18).
While polyurethane elastomers as a class have excellent tear strength and can be designed to the re~
quired modulus and elongation, not all polyurethane elastomers can meet the two requirements of low tempera-ture impact resistance and resistance to heat distorti~n.
fact, a polyurethane elastome~ based on poly(oxypropy-lene) glycol as ~he polymeric diol and 1,4-butanedlol the chain extender has not yet been used for flexible automoblle body parts because of the previous deficien-2~ ~ c~ies Or such an elastomer in these two areas. It is generally recognized ~N. E. ~ustad and R. G. Krawiec, Rubber~Age, No~. 1973, pp. 45-49) that elastomers based on poly(oxypropylene) glycols ha~e poorer low temperature properties than those based on poly(tetramethyleneeth~r) ~ glycol, another polyol used in polyurethane elastomers 1 ~ but higher in cost. One known way to improve the low I ~ - temperature properties is to increase the molecular weight o~ the polyol while keeping the mol ratios o~
- ; ingredients constant. U~fortunately, while the low ~ temperature properties are indeed lmproved~ the hardness .~

.,; .
, ,: .
. . . .

~ -

-3- ~ ~
and rigidity are normally lowered markedly. See Table IIg page 47 of the Rustad et al. article.
In copending application Serial No. 211,254, filed October 11, 1974 (corresponding to U. S. Patent 3,9155937~
October 28, 1975) we described a poly(oxypropylene) glycol based elastomer suitable for automobile flexible exterior body parts. Such a material can be prepared ~rom a polyol of approximately 1750 to 2500 molecular weight, methylene-bis(4-phenylisocyanate) and 1,4-butanediol, the molar ratio of butanediol to polyol being about 3.0:1 to about 9.0~
Our copending application was based on the fact that it was ~ost unexpected to be able to make hard elastomeræ with the neces~ary high and low temperature properties from poly(oxy-propylene) glycol.
While the specific formulation for a poly(oxy-propylene) glycol based elastomer necessary to achieve the proper combination o~ propert1es had not been de-scribed prior to our copending application, there had ap-peared a paper describing a similar concept applied to flexible automobile body parts using elastomers based on polgcaprolactone diol as the polyol. This paper, by F. ~. i Critchfield, J. V. Koleske and C. G. Seefried, Jr., wa~
presented at the Automobile Engineering Meeting of the Society of Automotive Engineers in Detroit, Michigan dur-ing the week of May 14-18, 1973. Summarizing their data on the polycaprolactone diol based elastomers, the authors stated ~Ifor automotive elastomer applications, thermo- ;
plastic polyurethanes based on an approximately 2000 ~n diol are more desirable since they show less modu-lus-temperature dependence in the use region.l' They also concluded: 'IApparently at similar hard segment con-~3~

,._

-4-centrations~ the molecular weight o~ the urethane polymer soft segment has a greater effect on the temper~ture dependence of physical properties than the molecular length o~ the hard segment sequences." They attrlbuted the unique properties of these materials to be the result of incompatibility on a microscopic scale be~ween the hard and soft segments. In turn, "Incompatibility quite probably is due to the molecular weight of the soft seg-ment being high enough to be immiscible in a thermodynamic sense with the hard segment."
Completely independently of the paper last mentioned abo~e~ we found~ as described in our copending application~ that polyurethane elastomers suitable for the preparation of fl~xible automobile exterior body parts may be obtained from the reaction of a mixture , :
comprising:
a) a polymeric diol selected from the group con-sisting of poly(oxypropylene~ glycol and ethylene oxide "tipped" poly(oxypropylene) glycol containing up to 10%
by weight of ethylene oxide and of molecular weight rom about 1750 to about 2500 (pre~erably about 2000) b) methylenebis(4-phenylisocyanate) c) 1,4-butanediol.
In the copending application the effect of the polyol molecular weight on ~he required properties was demonstrated. It was shown that polymer based on 1000 molecular weight polyol failed the low temperature impact and heat distortion tests wh.ile the polymer based on 2000 molecular weight polyol pas~ed both tests. The acceptable range of polyol molecular weight was shown to be 1750 to 2500. An elastomer prepared from 1500 _-:; 108~89S

-5-molecular weight polyol was not acceptable with respect to low temperature im~act while a pol~mer based on 3000 molecular weight polyol had lowered physical pro-perties. The latter result was believed to be due to separation of sof~ and hard phases early enough in the polymerization to immobilize reactive end groups and thereby inhibit chain extension EXPOSITION OF THE ~TION
Although the polymers described in the copending application are useful and can be handled with reasonable care, they do suffer from one deficiency, that of poor thermal stability at processing temperstures. I~ normal use this deficiency may not present a serious problem and may even go unnot-iced. However, since occasions may and often do arise in which material may be left in the barrel of an extruder or an injection moding machine for extended periods at elevated temperatures, it would be advantageous for a material to have superior thermal stability. In this way it would be possible to leave the material in the machines at temperature during short shutdowns and then resume operations with no cleanout and waste necessary. In addition it would insure that inferior parts ~ould not be produced because of thermally induced decomposition of the elastomer during the process.
m is is especially of concern when it is desired to use "regrind".
Unexpectedly we now have found that elastomers based on poly(oxypropylene)-poly(oxyethylene) glycol o~
oxyethylene group content 15~ (by weight) or hiGher possess significantly better thermal stability than those .
_~_ ,_~ 10818g5 _6-based on polyols containing 10% or less oxyethylene group content. Particularly preferred are polyols con-taining 30% or more oxyethylene group content, To our knowledge, this relationship between oxyethylene group content and thermal stability has not been described previously and is quite unexpected.
In addition we have found that this improvement ln thermal stability can be achieved with no sacrifice n the properties essential to automobile flexible body part use. In fact, slightly better strength properties appear to result from the use o~ polyols with higher oxyethylene group content.
Our invention, therefore, may be described in the following manner: ~ -15 ~ ~ Polyurethane elastomers suitable for the preparation of ~lexible automoblle e~terior body parts may be obtained from~the reaction~;of a mixture Comprising~
4j~ a poIy(oxypropy1ene)-po1y(oxyethy1ene) glycol o~molecular weight ~rom about 1750 to about ~20~ OOO~and containing~l5% to 50% oxyethylene group content by weight.~ ~
b)~methylenebis (~-phenylisocyanate) ; c) ;1,4-butanediol.
In order to study thermal stability the following 25~ ~ ~test~was devised. Polymer samples were molded into 2" x 0.125" x 0.125" tensile bars in a four cavity mold using s 1/2 oz. Newbury inJection molding machille at barrel ~and nozzle temperature of 415F. (213C.). A~ter several p~eces were molded, material was aIlowed to stand in the barrel o~ the machine ~or twenty mlnutes at temperature.
Then two more mol~ings were made (each molding produces _6-~ ,,, ~ . . . . . .
. ,~ .. .

--~ 1081895 four tensile bars). Tensile strength was measurcd on samples molded w~th and without this thermal treatment.
In this test typical elastomers of the invention retained at least twice as much of their original tensile strength as othe~rise similar elastomers made from poly(oxypropy-lene)-poly~oxyethylene) glycol containing lO~ or less oxyethylene group content.
The elastomers of the invention meet the require-ments for flexible exterior body parts for automobiles.
m ey have a hardness of about 40 to 55 Shore D, pre~erably 45 to 50 Shore D. They have an elongation greater than 270~, an ultimate tensile strength of about 2900 psi or greater and a Die C tear st~ength of 500 pli or greater.
Painted par+s made from these elastomers remain intact under a 5 MPH impact at -20F. To simulate the dynamic conditions involved in a 5 MPH impact at -20Fo a drop impact test system was developed. The unit con-sists basically of a vertical guide tube, a drop weight o~ appropriate design and associated instrumentation.
Polymers to be evaluated were molded into 21~ x

6~ x o.o8~ specimens, which were conditioned in an en-vironmental chamber to -20F~ and then fitted into two slots 3 inches apart so that the sample ~ormed an inverted "U" with a total flexed height of 2 inches. me sample was impactedat its center line with a force of 50 ft. lbs., the weight traveling at 5 ~H at lmpact.
Drop height above the top of thesample was 38 inches.
m e drop weight is an 18 inch long cylinder weighing 16 lbs. It is 2.5 inches in diameter for 16.5 inches of its length and then tapers to a blunt end, which is the . .

-7-striklng surface.
Polymers with inadequate low temperature impact resistance lnvariably fractured ~n this test whereas the polymers of the invention passed. This test correlates well with the automobile manufacturer's testing ~here full slze parts are made and mounted on a car or a portion ~r a car. After cooling to -20F., the full size part is hit with a pendulum weight which is traveling at 5 MPH.
Parts made from these elastomers also withstand paint oven temperatures of 250F. without ob~ectionable shrinkage or distortion. To evaluate materials for heat distortion characteristlcs, a sag resistance test (Heat Test O'S) was developed. ~e ap~aratus consistc of a Jig to hold a 2" x 6" x o.o8" in~ection molded sample in a horizontal plane. The sample is ~ounted with 4 inches : ~ suspended beyond the edge of the clamp. The ~ig with the æample is then placed in an oven -pre-heated at 250F.
rOr 1/2 hour. The amount o~ sag ls the di~ference ln height from the end of the s~ample to a horizontal plane ~ before and after exposure to heat. Experience with a material that was acceptable to automobile manufacturers has shown that polyurethane elastomers with a sag less than 2.0 inches by this test will per~orm satis~actorily in palnt bake ovens used to cure painted large automotive parts. The present elastomers meet this test.
Example 1 describes the preparation of nine elasto-~er s~mples ~rom 2000 ~olecular weight polyols of varying oxyethylene content and the use o~ the above test to ; ~tudy thermal stabillty. The data ~how the dramatic im-provement in khermal stability of the polymers when the

-8-g oxyethylene group content of the polyol exceeds 15~ and particularly when it exceeds 30%, Example 2 demonstrates the ability of an elastomer based on a 2000 molecular weight polyol containing 45%
by weight oxyethylene group content to pass both low temperature and heat sag requirements for automotive parts.
~ile the preferred molecular weight of the polyol is about 2000, it is recognized that the molecular weight may be somewhat below or above this figure and still give acceptable properties. m us the lower limit of molecular weight is about 1750 since Example 3 shows the utility o~
a rolyol o~ 1800 molecular weight and 30% by weight oxy- -ethylene group content.
~: :
Example 4 shows the utility of a 3000 molecular weight polyol containing 45% by weight oxyethylene content.
e upper limit for molecular weight is about 4000, although thermal stability of polymers derived ~rom these polyols is even more dependent on oxyethylene content.
20~ mu9, Example 5 demonstrates that 30~ oxyethylene group content in a 3000 molecular weight polyol does not provide the th~al stabi}ity which 45% oxyethylene group content does.
Example 6 shows that an elastomer with good pro-perties can be prepared rrom a 4000 molecular weight polyol containing 30~ by weight oxyethylene group con-~; ~ tent. Our pre~ious application showed that with untipped poly(oxypropylene) glycol even 3000 molecular weight gave an elastomer with unacceptable strength.
m e molar ratio o~ chain extender to polyol which _g_ .

., .,. ; .

may be used ranges from 3/1 to 12/1 depending on the mole-cular ~Jeight of the polyol. It ranges from 3/1 for a 1750 molecular weight polyol to 12/1 for a 4000 molecular weight polyol The preferred molar ratio of chain extender to polyol for a 2000 molecular weight polyol ranges from 4/1 to 6/1 with 5.0 to 5.5 being preferred.
The polyol may be either a "tipped" polyol in which a poly(ox~propylene) glycol is reacted further with ethylene oxide giving rise to oxyethylene group blocks on each end of the polyol or a more random poly(oxy-propylene~poly(oxyethylene) glycol in which the propylene oxide and ethylene oxide reactants are introduced to-gether or in alternating pcrtions. The preparation of both types of polyo' is described in "Polyurethanes:
Chemlstry and Technology", Part I. Chemistry, by J. H.
Saunders and K, C Fr~sch, Interscience, New ~ork, 1962, pp, 36-37. The technique of tipping is further described in "Advances in Urethane Science and Technology" by K.C.
Frisch and S.L. Reegan, Technomic Publishing Company, Westport, Conn., 1973~ pp. 188-193.
; m e polymers of examples 1-7 were prepared ~rom tipped polyols. In exa~ple 8, a random poly(oxypropylene)-poly(oyyethylene) glycol was employed and the results show that a thermally stable polymer results.` Thus, oxyethylene group content, regardless of position in the polyol, ls the major factor in i~roved thermal stability.
The oxyethylene group content of the polyol may range from 15~ to 50~ wlth the hi~her levels being pre-ferred for the hi~her molecular weight polyols. For a 2000 molecular weight polyol the preferred oxyethylene ,~ -10-08~895 -lOa-group content is 25-45~. m e NCO/OH ratio used to prepare the elastomers may range from 0.95 to 1.2 with 1 00 to 1.05 being preferred In our previous application we indicated a preference for preparing the elastomer by a prepolymer process in which the polyol is first reacted with the chain extender in a separate step. Although that pro-cess is still preferred,m~ny of the elastomers of this invention may be prepared by a "one-shot" technique in which the polyol, chain extender and diisocyanate are reacted together in one step. This is rendered possible by the high oxyethylene group content of the polyol which gives greater compatibility in a 'one shot" reaction.
In addition, the tip~ed polyols, by virtue of the high primary hydroxyl content are more reactive with di-isocyanates tending also to give better results in a "one shot" reaction. The preparation of an elastomer of this invention by a "one shot" reaction is deecribed in Example 7. -~
: ~ :

~: : " ' : ::
, -lOa--A catalyst may or may not be used as desired. Some examples of useful catalysts are N-methyl-morpholine, N-ethyl-morpholine, triethyl am~ne, trlethylene diamine (Dabco; trademark), N,N'-bis(2-hydroxypropyl)-2-methyl piperazine, dimethyl ethanol amine, tertlary ~mino alco~ols, tertiary ester amines, stannous octoate, dibutyl tin di-laurate and the like.
Exam~le 1 Nine elastomer sa~ples were prepared in the follow-lng manner.
The polyol or polyol blend (0.23 equivalent) was dried at 100C under vacuum (~ 2 mm. ~g) for one hour.
It was then cooled to 50C and 1~38 equiva}ents of 4~
methylenebis (phenyl socyanate) was added. The mixture then was heated at 80C. for two hours under dry nitrogen.
To a weighed portion of this prepolymer (1.08 equivalents) heated to 110C was added 1.03 equivalents of 1,4-butanediol at 60. The mixture was open cast and cured for 20 minutes at 163C.
It was then diced, dried for two hours at 110C.
and in~ection molded into 2" x 0.125" x 0.125" tensile bars in a ~our cavity mold. Molding was carried out in a 1/2 oz. Newbury injection molding machine at barrel and nozzle ~emperatures of 213C. (415F.) After several pieces were molded, material was allowed to stand in the barrel of the machine for twenty minutes at temperature.
m en two more moldings were made (each molding produces four tensile bars). Tensile strength was determined be-fore and after the thermal treatment and are recorded in Table I, along with the percentage of tensile strength .
. , .

::

108~895 : . :
retained.
The polyols and blends had the *ollowlng com-pos~tion. One polyol (Polyol A) was a 2000 molecular weight ethylene oxide tipped poly(oxypropylene) glycol containing 10~ by weight ethylene oxide. Another polyol (Polyol B) was a 2000 molecular weight ethylene oxide tipped poly(oxypropylene) glycol containing 45~ by weight ethylene oxide. Blends o~ the two polyols were also prepared in which the weight percent o~ ethylene oxide ranged from 15~ to 40% in 5% increments. An addi-tional sample was prepared ~rom an untipped poly(oxy-propylene) glycol.
Results o~ the thermal stability tests are as follows:
TABIE I
% Ethylene Oxide Original Tensile ~ter ~ Tensile in Polyol TensileThermal Treat- Retained ment 0 2800 ~ ~ :

308~ 2170 ~1 * degraded - could not mold.
The data shows the dramatic improvement in ther-~ ~ 30 mal stability of the polymer when ethylene oxide content .~ ~ o~ the polyol exceeds about 15~ and particularly when it exceeds about 30%.
~- Exam~le 2 -~ Nine hundred nlnety ~our (994 parts) o~ a 994 eq.
wt. poly(oxypropylene) gl~col tipped with 45~ by weight .. . . . . . .. . . . . .

ethylene oxl~e was dried for one hour at 100C and 2 mm. Hg vacuum. It was then cooled to 50C.under dry nitrogen and 950 parts o~ pure MDI was added. The mix-ture was then heated at 80C. ~or two hours.
A portion of this prepolymer (1855 parts) was then heated to 110C., and 268 parts of 1,4-butanediol at 60C. was added. The mixture l~aS well mixed for 20 seconds, open cast and cured for 20 minutes at 163C, The polymer was then diced, dried ~or 2 hours at llO~C. and injection molded into 2" x 6" x o.o80"
plaques.
Physical properties were as follows:
Hardness Shore D 54 100% Modulus 2140 Tensile Strength4030 Elongation 280 Die C Tear 750 mese plaques passed the low temperature impact test described previously. In addition, the heat sag according to "~eat test O'S" was 1-3/4 inches.
Example 3 In the same manner as described in ~xample 2, 225 parts of an 1800 molecular weight poly(oxypropylene) gly-col tipped with 30~ by weight ethylene oxide was converted to a prepolymer with 177 parts o~ pure MDI. A portlon o~
this prepolymer t379 parts) was then converted to the elastomer with 45 parts of 1~4-butanediol.
~ This polymer was in~ecition molded into tensile bars and tested for thermal stability as described in Example I. The results were as follows:

.

Initial After 20 min. ~ 415F
Shore D lIardness 45 100~ Modulus 1150 1130 300~ Modulus 2270 2070 Tensile Strength3720 3870 Elongation 440 490 , Exam~le 4 In the same manner as described in Example 2, 250 parts of a 2860 mol. wt. poly(oxypropylene) glycol tipped with 45~ by weight ethylene oxide ~as converted to a prepolymer with 222 parts of pure ~DI. A portion o~
this prepolymer (460 parts) was then converted to the elastomer with 68.4 parts of 1,4-butanediol.
mis polymer was injection molded into tensil~ -bars and tested ~or thermal stabillty as described in Example 1 except at a temperature of 430F. The re-sults were as followæ:
InitialA~ter 20 min. f~ 430F
Shore D Hardness 43 44 100% Modulus 1600 1580 300% Modulus 2540 2310 Tensile Strength3570 3260 Elongation 430 460 ExamPle ~
25~ In the same manner as described in Example 2, 250 parts o~ a 3040 molecular weight poly(oxypropylene) glycol tipped with 30% by weight ethylene oxide was con-verted to a prepolymer with 208 parts o~ pure MDI. A
portion of this prepolymer (450 parts) was then converted to the elastomer with 64.3 parts o~ 1,4-butanediol.
This polymer was in~ection molded into tensile bars and tested ~or thermal stabilit~ as described in Example 1 except at a temperature o~ 430F. The results were as ~ollows:
.

..... .. .. ... ..
..

~081895 ' -15-Initial After 20 min. r~ 430F, Shore D lIardne~s 43 30 100~ Modulus 1650 992 300~ Modulus 2430 ~
Tensile Stren~th 3500 1094 Elongation 500 143 Example 6 In the same manner as described in E,xample 2, 250 parts of a 3800 molecular weight poly(oxypropylene) glycol tipped with 30~ by weight ethylene oxide was con-verted to a prepolymer with 212 parts of pure MDI. A
portion of this prepolymer (450 parts) was then converted to the elastomer with 56,5 parts o~ 1,4-butanediol.
This polymer was injection molded into tensile bars. Physical properties were as follows:

Shore D Xardness ~7 100% Modulus 1760 300% Modulus 2660 Tensile Strength 3350 :20 Elongation 430 Example 7 Two-hundred ~ifty four (254) parts of a 1015 equivalent weight poly(oxypropylene) glycol tipped with 45% by weight ethylene oxide was dried for one hour at 100C. and 2 mm. Hg ~acuum. Anhydrous 1,4-butanediol (53 parts) was then added and the mixture was stirred and heated to 110C.
To 290 parts of this mixture at 110 was added 176 parts of pure ~I which had been heated to 70C. m e mixture was stirred vigorously and allowed to exotherm.
a temperature o~ 120C., the mi~ture was open cast and - cured for 20 min. at 163C.
The polymer was then granulated and dried for two hours at 110C. It was the~ in~ection molded into tensile bars and tested ~or thermal stability as de-scribed in Example 1. The results w~re as ~ollows:

_15-,'" :, " ' ~ , ' ,, ` . ' , Initial ~fter ~0 min. ~ 15F.
Shore D Hardness 45 44 1007' Modulus 1130 1000 300~ Modulus 19~0 1730 Tensile S~rength3~30 3350 Elongation 490 560 Exam~le 8 In the same manner as described in Example 2, 259 parts of a 2000 molecular ~Jeight poly(oxypropylene)-poly~oxyethylene) glycol containing ~5~ by weight random-ly distributed oxyethylene grollps was converted to a prepolymer with 202 parts of pure MDI. A portion of this prepolymer (44~ parts) was then converted to the elastomer with 5~ parts of l,~-butanediol.
mis polymer was injection molded into tensile bars and tested for thermal stability as described in Example 1. The results were as follows:
~ InitialAfter 20 min. f~ 415F.
100% Modulus 1030 970 300% Modulus 2490 2130 ~ensile 3580 3060 Elongation 370 370 e automobile flexible body parts, which are a desired end-product o~ this invention, are fabricated by injection molding using the already prepared poly-u~ethane elastomer as the molding material. In this method, the elastomer is made into small dice or pellets suita~le for feeding to injection molding machines. Using the same preformed material~ a part may also be prepared by extrusion techniques includin~ profile extrusion and sheet extrusion followed by vacuum forming.
Alternatively, the part may be ~ormed by the method termed "liquid reaction molding", in which the .
.

reactants are rapidly injected into a mold wherein they cure.to ~orm the shaped elastomeric article directly. In this method, the polyol, chain extender and diisocyanate may be reacted in one step (one shot method) or the polyol and diisocyanate may be pre-reacted and then in~ected along with the chain extender to form the molded artlcle (pre-polymer method).

` :

Claims

What is claimed is:

1. A thermally stable polyurethane elastomer which is a reaction product of: (a) a poly(oxypropylene)-poly(oxyethylene) glycol of molecular weight from about 1750 to about 4000 and containing 15% to 50% by weight oxyethylene groups; (b) methylenebis(4-phenylisocyanate);
and (c) 1,4-butanediol; the NCO/OH equivalents ratio being from 0.95 to 1.2 and the molar ratio of (c) to (a) being from 3/1 to 12/1, said elastomer having a hardness of about 40 to 55 Shore D, an elongation of greater than 270% an ultimate tensile strength of at least 2700 psi and a Die C tear strength of at least 500 pli, said elastomer displaying improved thermal stability as evidenced by its ability to retain at least twice as much of its original tensile strength, after exposure to a temperature of 415°F. for twenty minutes, as an otherwise similar elastomer in which (a) contains 10% or less oxyethylene groups.

2. A thermally stable polyurethane elastomer as in claim 1 in which the molecular weight of (a) is about 2000 and the oxyethylene group content is 30-45%.

3. A shaped article prepared from the thermally stable polyurethane elastomer of claim 1 and characterized, when having a thickness of 0.08 inch, by remaining intact under a 5 mile per hour impact at -20°F. and by having a sag of less than two inches as determined by the Heat Test O'S.