Curing method for polyether

Abstract

This invention relates to a novel curing method of oligomers, using metal triflates, and particularly to the curing of hydroxyl terminated elastomers to achieve crosslinked polymers. The method finds particular use as an alternative cure methodology to replace isocyanate curing. There is further provided a cured and crosslinked polymer binder, which is particularly suitable and compatible for use with energetic materials. ##STR00001##

Claims

1. A method of making a durable isocyanate-free paint comprising the steps of: forming an admixture of at least one hydroxy terminated oligomer, at least one epoxy terminated oligomer and at least one metal trifluoromethanesulfonate salt catalyst, wherein the admixture does not react with water, and wherein the admixture is formed in a substantial absence of solvent; and curing the resultant admixture at 30 to 85 C. substantially in the absence of a solvent to form a layer of the isocyanate-free paint, wherein the at least one hydroxy terminated oligomer is of formula (i) ##STR00013## wherein A is a monomer repeat unit, m is the average number of monomer repeat units in the range of from 5 to 100, x is in the range of from 2 to 20, and the at least one epoxy terminated oligomer is of formula (ii) ##STR00014## wherein B is a monomer repeat unit, n is the average number of monomer repeat units in the range of from 5 to 100, and y is in the range of from 2 to 20; and wherein at least one of the epoxy or hydroxy terminated oligomer comprises 5% to 10% w/w of an oligomer which has greater than 2 functional groups selected from the group consisting of hydroxyl and epoxy functional groups, so as to promote crosslinking in the final crosslinked polymer and wherein A and B are independently selected from monomer repeat units comprising hydrocarbyl, esters, carbonates, ethers, amides, aromatics, heterocyclic or copolymers comprising mixtures thereof, wherein the admixture further comprises at least one filler material, and wherein the at least one filler material is an energetic material.

2. A method according to claim 1, wherein at least one of the at least one hydroxy terminated oligomer and the at least one epoxy terminated oligomer comprises 5 to 10% w/w of an oligomer which has 3 to 5 functional groups selected from the group consisting of hydroxyl and epoxy functional groups.

3. A method according to claim 1, wherein the admixture comprises a further epoxy terminated oligomer, which contains an average of 2.5 to 4 epoxy groups per oligomer chain, and is present in the range of from 5-10% w/w.

4. A method according to claim 1 wherein the metal of the at least one metal catalyst is a lanthanide or group III metal.

5. A method according to claim 4 wherein the metal of the at least one metal catalyst is scandium or yttrium.

6. A method according to claim 1, wherein the curing step is carried out in the temperature range of from 40 to 85 C.

7. A method according to claim 6, wherein the curing step is carried out in the temperature range of from 40 to 60 C.

8. A method according to claim 1 wherein the catalyst is present in an amount of from 0.01% to 2% by mass of the admixturer.

9. A method according to claim 1 comprising uniformly dispersing the energetic material in said admixture.

10. A method according to claim 1, wherein the energetic material comprises ammonium dinitramide.

Description

EXPERIMENTAL

(1) A general reaction scheme is shown below in reaction scheme 1.

(2) ##STR00012##

Example 1 General Polymer Preparation

(3) The polyhydroxy terminated oligomers, as shown in Table 1, were typically dried in vacuo at 50 C. overnight whilst stirring. It was observed that if the catalyst was added to the polyhydroxy terminated oligomer as a solid and stirred that the dissolution times were slow and crystalline matter remained. Hence, in all subsequent preparations, the catalyst was dissolved in a minimum aliquot of acetone and stirred in to the oligomer to form a pre-admixture. The acetone was then removed by application of a vacuum.

(4) The epoxy terminated oligomer component was added to the oligomer/catalyst pre-admixture to form the admixture, which was stirred in vacuo at 50 C. for 10 minutes until the epoxy terminated oligomer had been thoroughly mixed. The samples were poured into PTFE moulds and cured for at least a week at 60 C.

(5) The crosslinking process and epoxy consumption was followed by dynamic stress rheology (DSR) and Fourier transform infrared spectroscopy respectively. The mechanical properties of the crosslinked cured material were studied using dynamic mechanical analysis (DMA) spectroscopy.

(6) Hydroxy Terminated Oligomer Variation

(7) Analysis of Gel Time

(8) Gel time is a function of both the crosslinker molecule and the catalyst concentration, and is defined at the time when the elastic modulus crosses the damping modulus. Dynamic Stress Rheology (DSR) measurements were used to follow gel times of curing HTPB at 60 C. The gel time of the system can be tailored to meet end use requirements.

(9) The progress of the gel reaction was monitored, for several different catalyst and oligomer A and B compounds using proton nuclear magnetic resonance spectroscopy (HNMR). It was observed that the proton on the secondary alcohol, generated from the ring opening of an epoxide, remains present on the spectra during the course of curing. This indicates that the secondary alcohol does not itself undergo reaction with a further epoxide ring at the same rate as that observed for primary hydroxyl groups.

(10) TABLE-US-00001 TABLE 1 Gel time of hydroxy terminated oligomers cured with epoxy Epikote 828 and europium TFMS catalyst. 1.sup.st order rate Shear Modulus Sample Gel time constant at 60 C./MPa Desmophen 1800 >24 0.00048 0.33 polycaprolactone >24 0.0011 0.18 polyhexamethylene phthalate 4.083333 0.0067 0.85 polyhexamethylene carbonate >24 0.00084 0.18 diol polyethylene-co-1,2-butylene Not 0.0067 1.17 measured HTPB 1.41 0.074 0.53 Polypropylene glycol 3.5 0.0067 0.17 polyethylene glycol block co- polymer polynimmo pp570 >24 0.00735 0.065 PolyGLYN Batch 3.24 20 0.001 0.85

(11) Table 1 summarises the typical 1.sup.st order curing rates (as measured by infrared spectroscopy) and the modulus of the final materials as measured using Dynamic Mechanical Analysis (DMA). The molar ratio of the polyhydroxy oligomer hydroxy end groups to epoxy end groups was 1:1, catalysed by 0.6% by mass Eu(TFMS).sub.3 and the cure was carried out at 60 C.

(12) The crosslinking reaction was found to vary on the polymer type. The curing process for oligomer units that comprise polyether monomeric units tended to be less efficient than that measured for the monomeric units containing polyesters, HTPB and polyalkanes. This may be due to the oxyphilic behaviour of the lanthanide catalysts. Coordination of the LnTFMS on to the ether linkage, of the polyether, may decrease the effectiveness of the catalyst and thereby decrease the rate of reaction.

(13) Comparison of Isocyanate and Epoxy Cross Linked Polymers

(14) Table 2, below, indicates the tensile testing data of the admixture of HTPB and epoxy Epikote 828 cured by the method according to the invention. For comparison a polymerisation which uses isocyanate is provided. As a further comparison plasticized and un-plasticized examples were also prepared, the plasticiser was bis(2-ethylhexyl) sebacate.

(15) TABLE-US-00002 TABLE 2 Comparison of tensile data of isocyanate cured polybutadiene with epoxy cured polybutadiene. Tensile Maximum Modulus/ Stress/ Break Material MPa MPa Strain/% HTPB + isophorone di-isocyanate 1.0 0.4 55 HTPB + isophorone di-isocyanate + 0.2 0.1 66 25% dioctyl sebacate plasticiser HTPB + Epikote 828 + ErTFMS 1.4 0.3 26 HTPB + Epikote 828 + ErTFMS + 1.1 0.2 22 25% dioctyl sebacate plasticiser

(16) The physical properties were measured using Instron tensile testing. It can be seen from the results in Table 2 above that the highest modulus and maximum stress are for the unplasticised materials. The strain at break is lower for the epoxy cured materials because they are more crosslinked than the isocyanate materials.

(17) The mechanical properties of cured elastomers over a range of temperatures were measured using dynamic mechanical analysis (DMA). The DMA traces of isophorone di-isocyanate cured HTPB (cured at 60 C. for one week) were found to be similar to the epoxy cured HTPB. Therefore methods of synthesis according to the invention provide polymers with similar mechanical properties to that of isocyanate cured polymers.

(18) The lanthanide trifluoromethane sulphonate catalysts have been shown to significantly accelerate the epoxy ring opening process compared to un-catalysed reactions. The gel time in the presence of La(TFMS).sub.3 (5.38 hours) was more than double that measured for dysprosium and thulium TFMS. However, for most of the catalysts, the gel time did not vary greatly; this reflects the similar chemistries exhibited across the lanthanide series. The elements at the latter end of the series, however, do appear to accelerate epoxy ring opening faster than those lanthanides at the beginning of the series. This might reflect changes in Lewis acidity caused by the lanthanide contraction as the atomic number is increased.

(19) Epoxy Variation

(20) The effect of different epoxide oligomer materials on the curing of HTPB in the presence of one particular catalyst EuTFMS was undertaken and the results are provided in Table 3, below.

(21) TABLE-US-00003 TABLE 3 Effect of epoxy crosslinking agent on the curing of HTPB Shear Hydroxy Gel time/ 1.sup.st order rate Modulus at Epoxy oligomer oligomer hours constant 60 C./MPa neopentyl glycol HTPB 7.5 0.098 0.08 diglycidyl ether trimethylolpropane HTPB 5.9 0.0054 0.23 triglycidyl ether Epikote 828 HTPB 3.4 0.0087 0.69

(22) The gel time and rate of reaction varies depending on the epoxide used. There is no linear relationship between the rate of consumption of the epoxy ring, (i.e. ring opening) versus the gel time rate. This may be due to complex interactions of hydroxy oligomer functionality, epoxy functionality and changes in diffusivity due to network formation i.e. crosslinking, during the cure. The epoxy and hydroxyl oligomers were curable in a desirable time period.

(23) HTPB Cure Using Variety of Lanthanide Triflates

(24) HTPB (containing 1% by mass of calco2246(antioxidant)) was dried in vacuo at 50 C. overnight. Epikote 828 (available from Aldrich) was added such that there was 1:1 mol equivalence of epoxy to hydroxyl groups. A selection of lanthanide metal triflate catalysts were added in an amount of 0.1 mmol equivalent of catalyst per g of HTPB/Epikote 828. The catalyst was dissolved in a minimum quantity of solvent prior to adding to the mixture, which was subsequently removed under vacuo. The admixtures were cured at 60 C. in a fan oven for 7 days. The mechanical properties as measured by DMA are compared to a rubber made from isophorone di-isocyanate (IPDI) and HTPB.

(25) TABLE-US-00004 TABLE 4 Optimisation of HTPB cure. Shear Shear Gel 1.sup.st Order Modulus Modulus Time/ rate Tg/ at at Crosslinker Catalyst Mass % hours constant C. 60 C./MPa 25 C./MPa Epikote Ce(TFMS).sub.3 0.64 3.17 0.0090 74 1.64 1.59 828 Epikote Dy(TFMS).sub.3 0.61 2.76 0.010 77 0.68 0.62 828 Epikote Sm(TFMS).sub.3 0.59 3.60 0.0087 77 0.69 0.63 828 Epikote Yb(TFMS).sub.3 0.53 4.19 0.0080 74 0.71 0.63 828 Epikote Tb(TFMS).sub.3 0.60 2.97 77 0.76 0.68 828 IPDI 68 0.41 0.40

(26) The above curing reactions were followed using both infrared spectroscopy (FTIR) and dynamic stress rheology (DSR).

(27) In the case of FTIR measurements, it was found that the resolution of the epoxy peak at 1250 cm.sup.3 was poor, hence the epoxy content was followed using the epoxy combination band at 4541-4510 cm.sup.3 in the near IR region.

(28) A typical dynamic stress rheology (DSR) plot of curing material at 60 C. revealed that the shear modulus of the formed cross linked polymer material increased rapidly within the first three hours. Despite using different catalysts, the gel times were similartypically 3-4 hours.

(29) Beyond the gel point, the polymer network does not flow. Hence the measurements in Table 4 above, suggest that there is too much catalyst in the reaction mixture for explosive and propellant formulation. For the purpose of energetic binder manufacture, a gel time of about 10 to 15 hours would be required. The gel time is easily controlled by decreasing the amount of catalyst, from the results it would appear that decreasing the catalyst concentration to 0.24% will increase the gel time to 15.5 hours.

(30) DMA indicates that the cured materials (for example Dy(TFMS).sub.3 catalysed curing of HTPB) have higher crosslink densities than that obtained for HTPB cured with IPDI (1:1 isocyanate to hydroxyl equivalence). This may be due to secondary hydroxyls participating in the crosslinking process.

(31) The material cured in the presence of Ce(TFMS).sub.3 is stiffer than the other five materials. This may be due to the cerium species catalysing the oxidative crosslinking of the polybutadiene backbone (possibly via the Ce(IV) salt rather than the Ce(III) salt). The material aged to a brown colour similar to that observed for aged un-stabilised isocyanate cured HTPB.

(32) Group III Metal Trifluoromethanesulfonate Catalysts

(33) HTPB (containing 1% by mass of calco2246(antioxidant)) was dried in vacuo at 50 C. overnight. Yttrium triflate and scandium triflate were added to catalyse the reaction between HTPB oligomer and Epikote 828, as per Table 5. The admixture provides a 1:1 mol equivalence of epoxy to hydroxyl groups.

(34) TABLE-US-00005 TABLE 5 Curing HTPB with different group III metal trifluoromethanesulfonate catalysts 1.sup.st Gel Order Shear Shear Mass Time/ rate Tg/ Modulus at Modulus at Catalyst % hours constant C. 60 C./MPa 25 C./MPa Y(TFMS).sub.3 0.26 1.11 0.0016 77 0.53 0.48 Sc(TFMS).sub.3 0.48 0.20 0.010 77 0.84 0.77 Sc(TFMS).sub.3 0.16 5.06 0.0021 77 0.57 0.51

(35) The above reactions were followed using both infrared spectroscopy (FTIR) and dynamic stress rheology (DSR). The catalytic activity of scandium triflate was greater than that of the lanthanide triflate catalysts, as indicated by the gel time and infrared spectroscopy. The consequence of this is that lower quantities of scandium triflate compared to lanthanide triflates are required to catalyse HTPB crosslinking in an equivalent amount of time.

(36) Energetic Polymer Curing Optimisation

(37) Energetic polymers have been specifically designed to possess a very high heat of combustion compared to traditional polymers (such as HTPB). Therefore, when an energetic composite undergoes reaction, the energetic binder adds more energy to the output. Three energetic polymers were investigated with regard to the curing procedure: Polyglyn (Glycidyl nitrate polymer), GAP (Glycidyl azide polymer) and Polynimmo (3-nitratomethyl-3-methyloxetane polymer).

(38) PolyNIMMO and polyGLYN (2 hydroxy terminated) possess hydroxy functionality of less than two hydroxyls per polymer chain. Hence, nominally di-epoxy species such as Epikote 828 will facilitate chain extension rather than crosslinking. Multi functional epoxy crosslinkers are preferred for such materials.

(39) PolyNIMMO

(40) Table 6 below summarises polyNIMMO curing attempts. An excess of epoxy was used for crosslinking. The mixes were cured at 60 C. in a fan oven for 7 days.

(41) TABLE-US-00006 TABLE 6 PolyNIMMO curing summary. 1st order DMA G Catalyst Gel reaction at 60 C./ Catalyst mass % Epoxy oligomer time rate (IR) MPa Sm(TFMS)3 0.59 Epikote 828 + triphenylol >24 0.0006 0.05 methane triglycidyl ether (1:1 based on epoxy mols) Sm(TFMS)3 0.59 triphenylolmethane 15.1 0.001 0.09 triglycidyl ether Yb(TFMS)3 0.53 triphenylolmethane >24 hrs 0.0018 0.42 triglycidyl ether Sm(TFMS)3 0.59 triphenylolmethane >24 hrs 0.26 triglycidyl ether Dy(TFMS)3 0.60 resorcinol diglycidyl ether >24 hrs 0.0017 Isocyanate cured 0.14 polyNIMMO

(42) The polynimmo does not possess di-hydroxyl functionality; therefore, admixtures prepared require the use of a higher functionalised epoxy oligomer. PolyNIMMO cured with a trifunctional isocyanate (Desmodur N100) exhibited lower crosslink densities that polyNIMMO cured with trifunctional epoxy in the presence of lanthanide triflates. That is to say, the crosslinking was more effective using the epoxy as the crosslinker rather than the isocyanate.

(43) PolyGLYN

(44) Two forms of PolyGLYN are availableoligomers with secondary hydroxyl end groups (e.g. Batch 3.24) or oligomers with secondary and primary hydroxyl end groups (e.g. Batch BX51).

(45) TABLE-US-00007 TABLE 7 PolyGLYN and triphenylolmethane triglycidyl ether, using Eu(TFMS).sub.3. 1st order DMA G DMA G Catalyst Gel reaction at 0 C./ at 60 C./ Hydroxyl oligomer mass % time rate (IR) MPa MPa PolyGLYN Batch 0.63 20.2 0.0010 0.43 0.1 3.24 (1 hydroxyl) PolyGLYN Batch 0.63 >24 0.00053 1.5 0.14 BX51 (2 hydroxyl) PolyGLYN Batch NA NA NA 1.6 0.3 BX51 - isocyanate cure

(46) Table 7 above, summarises the curing of different batches of polyGLYN with epoxy crosslinkers. The structure of the endgroups depends on the method of synthesis. PolyGLYN containing 0.6% by mass Eu(TFMS).sub.3 catalyst was mixed under vacuum with triphenylolmethane triglycidyl ether in a ratio of 1.1 mols epoxy to 1 mol of polyGLYN hydroxyl group. The mixes were cured at 60 C., in a fan oven, for 7 days.

(47) The mechanical properties of the fully cured materials as shown in Tables 1-7 compare well with isocyanate cured samples. The results clearly show that hydroxy terminated oligomers crosslinked with epoxy terminated oligomers in the presence of a metal triflate catalyst used in a method according to the invention, provide crosslinked polymers that have similar physical properties to the same hydroxyl terminated oligomers when cured with isocyanates. The method according to the invention provides a less toxic, more cost effective route to cross linking hydroxyl terminated oligomers.

(48) The results have further shown that the use of at least one metal triflate in a method according to the invention provide a means of synthesising crosslinked binders which are suitable for use with explosive materials.