Structure-property relationship of HTPB-based propellants. II. Formulation tailoring for better mechanical properties

There has been a constant endeavor to improve the mechanical properties of hydroxylterminated polybutadiene (HTPB) -based composite solid propellants. A systematic study has been conducted on different batches of HTPB resins with varying molecular weights and hydroxyl values. Propellant formulation experiments were conducted wherein the ratio of chain extender to crosslinker was systematically varied, with a view to achieve the maximum possible strain capability and moderately high tensile strength, keeping all other parameters constant. The influence of increasing hydroxyl content from trimethylolpropane at the expense of hydroxyl content from butanediol, on the mechanical properties of the finished propellant, has been depicted on 3-dimensional graphs. The isoproperty lines, plotted as a triangular chart with the percentage hydroxyl contents from the three constituents, can be used to arrive at the suitable formulation for a specified application depending upon the OH value of the resin. HTPB resins with high molecular weight, low functionality, and low hydroxyl value require higher levels of trifunctional curing agent and higher NCO / OH ratios to obtain outstanding mechanical properties, especially elastic properties, compared to low molecular weight, high functionality resins. The impact of hard and soft segment domain structure on the mechanical behavior of the cured systems is more pronounced in the low molecular weight resin formulations due to the higher hard segment content compared to those attainable in high molecular weight resin formulations. © 1993 John Wiley & Sons, Inc.     

Functionality distribution and crosslink density of hydroxyl-terminated polybutadiene

A novel approach is proposed for estimating the average molecular weight between crosslinks (M?_c) from the functionality distribution of hydroxyl-terminated polybutadiene ( HTPB ). The functionality distribution of four free-radically polymerised HTPB prepolymers of varying hydroxyl content and molecular weight was determined by a combination of preparative and analytical gel permeation chromatography. The gumstock properties of the samples cured with stoichiometric amounts of toluene diisocyanate were not correlatable with the relative amounts of difunctional chain extender and multifunctional crosslinker present, unlike the case of HTPB with similar hydroxyl content and molecular weight. However, the mechanical properties and sol content could be correlated with the average molecular weight between crosslink sites, M?_c, of the cured polymer. The M?_c values derived by our method compare well with those of classical methods, and the observed differences are attributed to segmental entanglement. These M?_c values give consistently good correlation with all the gumstock properties, confirming the validity of our approach and the soundness of the techniques developed for the determination of the functionality distribution of HTPB .     

The effect of varying molecular weight distribution on the properties of binders

Work is reported on the effect of variation of molecular weight distribution on the properties of binder stocks. This work used three prepolymers (polypropylene glycol 1000, 2000, and 4000 and polypropylene glycol 400, 1000, and 2000) of different molecular weight and known molecular weight distribution. The prepolymer of the middle molecular weight was employed as standard, and stocks were prepared from it. To this prepolymer was added some of the high and low molecular weight prepolymers in proportions such that the crosslink density of stocks made from the mixture was the same as that of stocks made from the middle molecular weight prepolymer alone. Two series were prepared with different crosslinking agents. The proportions of high and low molecular weight prepolymer were successively increased until finally none of the middle molecular weight remained. Heterogeneity indexes of the glycols and their mixtures were measured by gel permeation chromatography. Elongation at break, modulus of elasticity, and extent of solvent swelling were determined on the crosslinked stocks. Properties which vary with crosslink density were found to show decreasing values with increasing heterogeneity index even though the stocks were formulated to a constant crosslink density. Narrow-distribution stocks reach the maximum degree of cure faster than the broader-distribution stocks. Infrared and thermal analysis confirm that monodisperse polymer has a greater extent of reaction than heterodisperse polymer. Results showed that the variation in elongation at break to be expected because of lot-to-lot variations in heterogeneity index is probably not greater than the experimental variation in the elongation test below a heterogeneity index of 1.5. Above 1.5, however, if the effects observed are entirely due to variation in heterogeneity index, lot-to-lot variations in molecular weight distribution cannot be ignored. An explanation is presented based on the varying ability of prepolymer molecules of different size to diffuse through uniform mesh openings resulting in lower final extents of reaction for broad distribution material.     

阴离子端羟基聚丁二烯的表征及其固化物的力学性能

简述了以硅烷保护羟基的烷基锂为引发剂,环己烷为溶剂合成丁羟胶的聚合方法;通过IR、^1H—NMR和GPC等方法研究了阴离子端羟基聚丁二烯主链结构、分子质量分布、官能度及其分布;对其固化物力学性能进行了评价。研究结果表明,阴离子端羟基聚丁二烯的1,4-结构含量高达约90％、分子质量分布指数≤1．05、官能度接近2,其固化物具有优异的力学性能。     

Chain-extended polyurethanes—Synthesis and characterization

Thermoset polyurethane (PU) elastomers were prepared using hydroxyl-terminated polybutadiene (HTPB), toluene diisocyanate (TDI) and hexamethylene diisocyanate (HMDI). The effects of various diamines and diols on the mechanical and thermal properties of polyurethanes are discussed. The average molecular weight between crosslinks (M?/_c) was determined by swelling studies. The properties imparted by the extenders are explained on the basis of the groups present in the diamines and the number of methylene carbons in the diols.     

Characterization studies on aging properties of acetyl ferrocene containing HTPB‐based elastomers

In composite solid propellants, low‐molecular‐weight species such as burning rate catalysts, plasticizer, etc. which migrate into liner and thermal insulation layers during curing and storage invariably result in poor mechanical and ballistic properties of the propellants. In the present study, the migration of the burning rate catalyst, acetyl ferrocene, was investigated spectrophotometrically (UV–visible) by evaluating the extent of hindrance to such migration after applying a barrier (liner) of various crosslink densities between the additive (HTPB‐TDI‐plasticizer–acetyl ferrocene) and nonadditive (HTPB‐TDI) gumstocks replicating the propellant and insulating layer, respectively. Enhancing the crosslink densities of liner via a trifunctional aziridine crosslinking agent inhibited migration. The aging of additive gumstock was done at 60°C and its mechanical properties and extent of acetyl ferrocene migration were also evaluated and analyzed. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2538–2545, 2006     

Synthesis and structure–property relationships of UV-curable urethane prepolymers with hard–soft–hard blocks

UV-curable urethane prepolymers derived from diisocyanates, rigid diols, and polypropylene glycols of different molecular weights and end-capped with 2-hydroxylethyl methacrylate were prepared by stepwise addition reactions. These prepolymers have the common structure, HEMA-hard segment-soft segment-hard segment-HEMA. As the weight ratio of soft segment to hard segment and thus the rigidity varied, the cured films from these prepolymers exhibited a wide range of mechanical and other key properties such as oxygen permeability.     

Studies on urethane–allophanate networks based on hydroxyl‐terminated polybutadiene: Modeling of network parameters and correlation to mechanical properties

Hydroxyl‐terminated polybutadiene (HTPB)‐based allophanate–urethane networks were prepared by reacting HTPB with di‐isocyanates, such as toluene–di‐isocyanate (TDI), isophorone–di‐isocyanate (IPDI), and 4,4′‐di(socyanatocyclohexyl)methane (H₁₂MDI) at stoichiometric ratios (r‐values) ranging from 1.0 to 1.5. The networks were characterized for mechanical and swell properties. The network parameters, such as “X,” which is the fraction of urethane groups involved in the allophanate formation, and effective chain length (L_x) were calculated from experimental crosslink density values determined from swell data, using α‐model equations developed by Marsh. Excellent linear correlations were obtained between mechanical properties and the calculated network parameters. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2986–2994, 2006     

Poly(ester‐urethane)s synthesized using polyoxalate diols

Amorphous poly(ester‐urethane)s (PEUs) were synthesized by reacting polyoxalate diols (PODs), which are oligoester diols prepared from condensation polymerization of dimethyl oxalate (DMO) and alkane diols, with 4, 4′‐diphenylmethane diisocyanate (MDI) and propylene diamine (PDA), a chain extender. Their structure–property relationships were studied, mainly focused on effects of molecular weight and alkylene chain length of the POD. The synthesized PEUs were buried in compost soil at 30°C and incubated to conveniently evaluate their biodegradability. Their hydrolytic characteristics were also examined, and what made poly(oxalate‐urethane) (POU) biodegradable was discussed. Poly(oxalate carbonate‐urethane) (POCU), which can be produced adding a polycarbonate diol (PCD) into the POD and then copolymerizing them with DMO, provided biodegradable polyurethanes with mechanical properties appropriate for practical uses. In addition, the microstructure of these copolyurethanes was characterized. POLYM. ENG. SCI., 45:163–173, 2005. © 2005 Society of Plastics Engineers.     

高拉伸强度高伸长率丁羟弹性体的研究进展

综述了影响丁羟弹性体力学性能的几种关键因素,包括固化参数、固化剂类型、扩链剂类型等,对比了几种提高丁羟弹性体力学性能的方法及其研究进展。提出了通过合成具有不同官能度的窄分子质量分布的丁羟液体橡胶,并进行复配研究,将是制备高拉仲强度、高伸长率丁羟弹性体的研究方向。     

Thermoplastic urethane elastomers. I. Effects of soft-segment variations

A series of thermoplastic urethane elastomers with soft segments of varying sequence length was prepared and their dynamic mechanical properties were characterized over a wide temperature range. The polymers were prepared using various molecular weight polycaprolactone diols as the soft segment and 4,4′-diphenylmethane diisocyanate and 1,4-butanediol as the hard segment. The urethane elastomer exhibited soft-segment crystallization when a polycaprolactone diol greater than 3000 M?_n was used. The glass transition temperature of these materials progressively shifted to lower temperatures as the chain length of the soft segment was increased. This dependence was interpreted in terms of a molecular weight relationship similar to that associated with amorphous homopolymers. The dynamic mechanical properties of these polyurethanes appear to be consistent with responses observed for compatible copolymers.     

浇注型端羟基聚丁二烯聚氨酯弹性体的合成及性能研究

采用不同分子量的端羟基聚丁二烯（ＨＴＰＢ）和甲苯二异氰酸酯为主要原料合成预聚物，以Ｎ，Ｎ－Ｍ（２－羟丙基）苯胺为扩链剂制备了浇注型聚氨酯弹性体，并着重研究了预聚体中游离异氰酸酯基含量、扩链剂用量、ＨＴＰＢ分子量以及不同分子量ＨＴＰＢ并用和扩链剂并用对弹性体力学性能的影响，还对弹性体的结构与形态进行了初步分析和探讨。结果表明，预聚体中游离异氰酸酯基含量为９．０％时，拉伸强度最大，且综合性能最佳；扩链系数为０．８９时，拉伸强度、定伸应力、硬度出现最大值；ＨＴＰＢ分子量增大，弹性体力学性能有下降趋势，不同分子量ＨＴＰＢ并用时分子量大的ＨＴＰＢ增多，力学性能下降；当ＨＴＰＢＭｎ＝３１００，游离异氰酸酯基含量为９．０％，扩链系数为０．８９时，弹性体综合性能最佳。电子显微镜照片显示ＨＴＰＢ型聚氨酯有微相分离，且软硬键段分布不规整。     

Synthesis of urethane oils from waste poly(ethylene terephthalate) bottles

Waste poly(ethylene terephthalate) (PET) bottles were glycolyzed by propylene glycol (PG) at a weight ratio of PET to PG of 37.5 : 62.5 using zinc acetate as a catalyst. The glycolyzed product, consisting of oligomeric diols with a number‐average molecular weight range of 458–844, was obtained. It was further reacted with soybean oil and toluene diisocyanate to obtain urethane oils at hydroxyl to isocyanate ratios from 1 : 1 to 1 : 0.7, with and without methanol acting as a blocking agent. All the synthesized urethane oils were yellowish, transparent, low‐viscosity liquids of low molecular weights. A lower diisocyanate content and the presence of a blocking agent resulted in higher viscosity, higher molecular weight, and shorter drying time. The films of all synthesized urethane oils exhibited good hardness and adhesion. They also showed excellent water and acid resistance but only fair alkali resistance. However, these prepared urethane oils had lower flexibility and poorer wear resistance compared to those of the commercial urethane oil. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3040–3045, 2004     

IPDI型水性聚氨酯预聚体水分散性及乳液成膜物力学性能的研究

以自制不同相对分子质最的聚已二酸-新戊二醇酯二醇(PNA型聚酯二醇)和异佛尔酮二异氰酸酯(IP-DI)为主要原料,二羟甲基丙酸(DMPA)为亲水单体,水合肼为扩链剂,制备了IPDI型的水性聚氨酯乳液.研究了PNA聚酯二醇相对分子质量对IPDI型预聚体黏度和水分散性的影响.实验结果表明,将PNA聚酯二醇的相对分子质量控制在1 400以下时,用低相对分子质量的PNA与IPDI的预聚体具有黏度低、易于水分散的特点,可以实现水分散时不加或少加有机溶剂降黏的目的;利用万能材料试验机、XRD、FT-IR和差示扫描量热仪等仪器对所得乳液成膜物的力学性能进行了测试,结果表明:用低相对分子质量的(＜1 400)PNA合成的IPDI型乳液的成膜物力学性能可以满足应用的要求.     

Engineering plastics from lignin. VI. Structure–property relationships of PEG-containing polyurethane networks

Hydroxypropyl lignin-based thermosetting polyurethanes were synthesized with excess hexamethylene diisocyanate (HDI) and tolylene diisocyanate (TDI) by solution casting. Four polyethylene glycols (PEG) of molecular weight 400, 600, 1000, and 4000 were mixed with lignin polyol to incorporate different proportions of soft segment into the network prior to crosslinking. Neither thermal nor mechanical and limited small angle x-ray scattering (SAXS) analysis provided distinct evidence for phase separation and microstructure formation. The study examines the effect of the soft segment in relation to chain length and weight contribution on the thermal and mechanical properties of the final networks. A significant sensitivity of glass transition temperature (T_g), of swelling in DMF, and of the mechanical properties to soft segment content was observed. Some of this sensitivity must, however, be attributed to differences in crosslink density since the polyol to diisocyanate weight ratio was kept constant throughout the synthesis series. The magnitude of the change of the different properties was found to be influenced by both glycol content and glycol molecular weight. The T_g of the network decreased from 105°C to as low as 38°C (HDI), and from 158°C to 70°C (TDI), with incorporation of up to 17.8% glycol, and it was greater with lower molecular weight glycols than with higher ones at any weight fraction. Swelling in DMF increased as expected with soft segment content. Mechanical properties were affected most if HDI and lower molecular weight glycols were used. The uniformity in structure, reduction in brittleness, and considerable improvement in mechanical properties with inclusion of minor PEG contents indicates that lignin-based network polyurethanes can be synthesized with controllable performance characteristics.     

Size exclusion chrormatographic studies on the urethane reaction in hydroxyl-terminated polymers

Size exclusion chromatography (SEC) has been used to study the influence of molecular weight on the reactivity of the hydroxyl group in free radically polymerised hydroxyl-terminated polybutadiene (HTPB). The HTPB prepolymer was converted into a UV-absorbing urethane derivative by reaction with phenylisocyanate and the degree of conversion was followed from the UV/DRI (Differential Refractive Index) signal of the derivative using dual detector SEC. Though the overall reaction levelled off at about 70% conversion at room temperature, the time taken by different molecular weight species to attain this level was different. The computed rate constants were larger for the higher molecular weights in a given sample. This is attributed to the polyfunctionality of the higher molecular weight species.     

Effects of number-average molecular weight of liquid hydroxyl-terminated polybutadiene on physical properties of the elastomer

The relationship between the molecular weight of liquid hydroxyl-terminated polybutadiene (HTPB) and the physical properties of the elastomers were investigated. HTPB having various molecular weights were cured with 4,4′-diphenylmethane diisocyanate (MDI) by the one-shot method and the physical properties of these elastomers obtained were measured. The tensile strenght, modulus, tear resistance, and the hardness decreased with increase in the molecular weight of HTPB, and this phenomenon was remarkable in the low molecular weight HTPB (below about 3000). On the other hand, the ultimate elongation increased linearly with increase in the molecular weight of HTPB. The effects of incorporating a short-chain diol, N,N-bis(2-hydroxypropyl)aniline, on the physical properties on the elastomers are also discussed.     

Effect of HTPB structure on prepolymer characteristics and on mechanical properties of polybutadiene-based polyurethanes

Oligobutadiene-based prepolymers and polyurethanes (PU) were prepared from hydroxyl terminated polybutadiene (HTPB), three diisocyanates (aromatic, alicyclic and aliphatic) and two chain extenders (containing hydroxyl or amino groups). The free NCO content in the prepolymers varied systematically and the characteristic of these products were interpreted in terms of NCO group reactivity, propagation reactions and HTPB functionality. The mechanical properties of the polyurethane elastomers derived from the prepolymers were discussed in terms of hard and soft segment structures.     

丁羟型聚氨酯弹性体力学性能研究

研究了以不同分子质量的丁羟（HTPB）、二异氰酸酯和固化剂为主要原料制备的PU弹性体的力学性能。结果表明,PU弹性体的力学性能随丁羟相对分子质量的增大而提高;R（n-NCO／n-OH）值达到8时,PU的力学性能最佳;用4种固化剂固化PU弹性体,结果表明复配的固化剂固化的PU弹性体力学性能最好;固化温度越高、时间越长,PU弹性体的力学性能越好。     

Network mechanical properties of amine-cured epoxies

The technique of Impulse Viscoelasticity was used to characterize the network mechanical properties of amine-cured epoxies during cure. The effects of amine molecular weight, functionality and stoichiometry were investigated. Among the properties which were obtained were the equilibrium tensile modulus, gelation time, cure and thermal stresses, volumetric changes during cure, glass transition temperature, thermal expansion coefficient, and molecular weight between cress-links. It was found that these networks cured elastically and agreed closely with the predictions of rubber elasticity theory over a wide range of crosslink densities.