LYU Shuo,QIANG Hongfu,CHEN Hui,WANG Lu,ZHANG Lijun,HU Runzhi

(1.Rocket Force University of Engineering,Xi’an 710025,China;2.Xi’an Aerospace Information Institute,Xi’an 710025,China;3.Academy of Aerospace Solid Propulsion Technology,Xi’an 710025,China;4.Xi 'an Aerospace Chemical Power Co.,Ltd.,Xi'an 710025,China)

Abstract：3D printing technology has significant advantages in the preparation of solid propellant grain with a complex structure and can improve manufacturing efficiency and process safety.The modified HTPB propellant slurry used for 3D-printing was prepared by adding a new UV-curable polyether adhesive,and the UV-curable polyether HTPB propellant grains with complex structures were fabricated by an explosion-proof 3D printer.In order to characterize the compatibility between the UV-curable polyether adhesive and AP or Al,the decomposition temperature of the UV-curable polyether adhesive with the addition of AP or Al was tested by DSC.The UV-curable polyether modified HTPB propellant formulations were determined by using the UV light source tests,the UV-curable depth tests,and propellant extrusion tests.The properties of UV-curable polyether propellant grains were characterized by tensile tests, porosity tests,density tests and viscosity test. The results show that the UV-curable polyether adhesive is quite compatible to AP and Al; the tensile strength,the maximum elongation, the porosity and the density of the 3D printed UV-curable polyether HTPB propellant grains are 0.299 MPa,22.4%,6.8% and 1.62g/cm3,respectively.

Key words：HTPB solid propellant;3D-printing technology; materials extrusion;UV-curable polyether

0 Introduction

The HTPB composite solid propellant is widely used as the main propulsion working medium in various strategic missile or tactical missile.Due to the limitation of subtractive manufacturing processing,in the traditional cast-curing manufacturing processing of HTPB propellant grain,the mandrel with a desired shape must be used as a process tool to form the particular ports of a propellant grain,aiming to increase the burning area of the grain.It is apparent that some complex propellant grains with submerged ports and/or multiple formulations gradient etc.are difficult to be fabricated or produced in batches by using the traditional cast-curing manufacturing processing.3D-printing technology,a disruptive technology in the field of energetic materials,may provide a possible processing solution for manufacturing propellant grains.Considering the inhered hazards,the reported material extrusion technology (3D-printing technology) is relatively popular in the fabrication of composite solid propellant,which represents the main investigation trends in the field of manufacturing technology for composite solid propellant.

The application of 3D-printing would bring about four advantages:1) to get a higher thrust or a predesigned variable thrust;2) to shorten the fabrication cycle and to increase the efficiency of design-fabrication-test;3) to remove risky processes such as grain transfer and reduce exposure time of slurry/grain;4) to achieve in-situ 3D-printing of rocket components all-in-one covering a grain,insulations,a case,nozzles,injectors,etc.,associating with a ultimate target to improve the reliability of solid rocket motor.

When the traditional HTPB propellant slurry is adopted to extrude directly,the extruded slurry layer requires a relatively longer time to maintain shape fidelity before a new layer deposition,which decreases the fabrication efficiency greatly and might bring a risk.To address this issue,the traditional HTPB propellant formulations should be modified by adding some compositions such as photo sensitizer or heat sensitizer,to facilitate slurry layer precuring within 5～30 s.However,there have not been many studies on the challenge.In this paper,a novel UV-curable polyether adhesive is adopted to modify the traditional HTPB formulations,expecting that the designed UV-curable polyether HTPB propellant grains could be 3D-printed with high quality.

1 Explosion-proof 3D-printer and formulations

1.1 Printing principle and explosion-proof 3D-printer

Material extrusion (ME) is a typical type of manufacturing technique,extensively used for the rapid prototyping and manufacturing of various materials.The material extrusion process for the soft material is quite similar to the conventional slurry casting process.Therefore,the material extrusion techniques are selected to additively manufacture composite solid propellant.

The 3D printing principle for the material extrusion technique is that the soft composite solid propellant slurry is pushed through a nozzle,which can be heated by water and/or vibrated by a ultrasonic oscillation,and then deposited gently down on the print bed layer by layer until the physical grain is printed as designed (Fig.1).

Fig.1 Principle of material extrusion 3D-printer

The photo-polymerization is a light-processing technology,by which the liquid or semi-liquid substances are solidified into a solid state through light irradiation in a certain wavelength,accompanying with photo-induced polymerization and crosslinking reaction.

Fig.2 Explosion-proof 3D-printer

The explosion-proof 3D-printer is mainly composed of forming platform,extrusion system and photocuring system (Fig.2).The extrusion system includes plunger sub-system,slurry barrels,print-heads and computer system.During 3D-printing,the predesigned models are sliced in the computer system,commanding the extrusion system to 3D-print the grains;the vertical movement of the forming platform and the correct lighting of the photocuring system are coordinated.

1.2 Formulation and materials

By adding a lab-made UV-curable polyether adhesive,the traditional HTPB based propellant formulations are modified,as shown in Table 1.The UV-curable polyether adhesive is vital for the 3D-printing solid propellant,due to its outstanding photocuring shape fidelity.The other ingredients of the modified formulations are identical to that of the traditional formulations,such as HTPB,TDI,Al and AP.

The UV-curable polyether adhesive is a mixture of polyether-toluene-diisocyanate prepolymer,hydroxypropyl acrylate (HPA),photo initiator,1,4-butanediol,etc.The polyether-toluene-diisocyanate prepolymer with 4%～6% isocyanate radical (NCO),are successfully prepared by controlling the raw material molar ratio of polyether polyol and toluene diisocyanate.

Table 1 UV-curable polyether modified HTPB solid propellant formulation

2 Testing and characterization

2.1 Compatibility characterization

Compatibility is important for estimating if there are potential risks in the process of design,production and storage.The purpose of the DSC characterization is to investigate the mutual effect between the UV-curable polyether adhesive and the main solid fillers such as AP and Al,particularly the change in thermal decomposition history from the introduction of the polyether adhesive.The relative mass proportions of the polyether adhesive and Al/AP are 1︰1 for the prepared DSC samples.

2.2 Ultraviolet illumination intensity test

The selection of ultraviolet light power is significant for the illumination intensity,which is closely associated with the 3D-printing efficiency and the quality of the 3D-printed grains.The ultraviolet sources with power 5 W,40 W and 60 W are selected to illuminate the UV-curable solid propellant specimens,to sieve the suitable illumination intensity by recording the quality of illuminated specimens and corresponding illumination time.

2.3 UV-curable depth test

There are various ingredients in the propellant formulations,however,the solid fillers such as AP and Al are key factors to the UV-curable performance because the solid fillers affect the propagation of ultraviolet light.AP appears as white crystalline powder,with relatively favorable light transmission,whereas the Al powder (grey) and burning rate catalyst PbO(orange) have a remarkable influence on the UV-curable performance of the propellant slurry.Moreover,the color of the propellant turns deeper with increasing Al and PbOcontent,which would have the greater impact on the UV-curable depth of the propellant slurry,theoretically.To investigate the effect of Al/PbOon the UV-curable depth practically and further figure out the optimal Al/ PbOcontent,the UV-curable depth tests are conducted under the same ultraviolet illumination conditions.

2.4 Propellant slurry extrusion test

The propellant slurry extrusion tests reveal the law how the slurry extrusion quality varies with the solids content,suggesting the optimal sizes of print-head for a certain solids content.In the slurry extrusion test,the 2 mm print-head is employed to extrude propellant slurry with different solids content,and the corresponding extrusion quality is recorded and analyzed.In addition,rotating cylinder viscometer is used to test the viscosity of 50 g propellant slurry with 80% solids content under the thermal condition of 50 ℃,to observe the pot life of the 3D printing formulation.

2.5 Curing parameter test

Curing parameter,defined as NCO/OH molar ratio,plays a key role during the formulation design.For curing parameter test,The 3D-printed propellant with 80% solids content are post-cured at 50 ℃ for 7 d.Through the adjustment of curing parameter (=1.0,1.1,1.15,1.2),the curing performance of propellant slurry is observed and investigated,with the purpose of determining the optimal curing parameter.

2.6 Tensile test

The tensile tests of 3D-printed solid propellant grains are performed to figure out whether tensile strength and the maximum elongation meet the application requirements or not.The dumbbell-shaped specimens are fabricated from the 3D-printed grains (Fig.3),post-cured in an incubator with 50 ℃for 7 d after 3D-printing of the formulations in Table 1.

Fig.3 3D-printed grains with intricate geometry

Several groups of dumbbell-shaped specimen are tested on the universal testing machine according to QJ924—85 unidirectional tensile test of composite solid propellant,to acquire the maximum load,the tensile strength,the maximum elongation,the breaking elongation,the modulus and the breaking strength.

2.7 Porosity test and density test

To further understand the mesostructure difference between the 3D-printed grains and the cast-cured grains,a Sky Scan 1172 high precision micro-CT instrument is employed to analyze and compare the porosity of 3D-printed grains and typically cast-cured grains.The size of fabricated specimens is 3 mm×3 mm×6 mm,as shown in Fig.4.

Fig.4 Micro-CT specimens: the upper is 3D-printed;the lower is cast-cured

3 Results and discussion

3.1 Compatibility of polyether adhesive and Al/AP

As shown in Fig.5 and Table 2,the first decomposition temperature of polyether adhesive is 257.1 ℃,while the corresponding temperature of the mixture (mass ratio 1︰1) of polyether adhesive and Al is 254.2 ℃.The second decomposition temperature of polyether adhesive is 288.8 ℃,while the corresponding temperature of the mixture (mass ratio 1︰1) of polyether adhesive and Al is 291.3 ℃.Since the decomposition temperature of the polyether adhesive compromises little with the addition of Al,it suggests that the compatibility of polyether adhesive with Al meets the design requirement of solid propellant.

Table 2 The decomposition temperature peak for polyether adhesive,mixture of polyether adhesive and Al(mass ratio 1︰1)

Fig.5 DSC curves of polyether adhesive,Al,mixture ofpolyether adhesive and Al (mass ratio1︰1)

The DSC curves in Fig.6 and Table 3 show that the low-temperature decomposition peak of AP decreases to 200.0 ℃ from the original 241.7 ℃ after mixing with polyether adhesive (mass ratio:1︰1),while the high-temperature decomposition peak of AP increases to 310.0 ℃ from 290.9 ℃.The drop of AP low-temperature decomposition peak benefit AP burning and decomposition;the shift of AP high-temperature decomposition peak has little influence on the AP combustion since AP has already been burning at 310.0 ℃.Therefore,the polyether adhesive has satisfying compatibility with AP below 310 ℃,indicating that the polyether adhesive can be applied in 3D-printing propellant formulation.

Table 3 The decomposition temperature peak for AP,mixture of polyether adhesive and AP (mass ratio 1︰1)

Fig.6 DSC curves of polyether adhesive,AP,mixture ofpolyether adhesive and AP (mass ratio 1︰1)

3.2 Effect of ultraviolet illumination intensity on propellant curing

The ultraviolet illumination intensity tests reveal,illuminating the propellant specimens under the identical conditions,that the exposure of 5 W lasts 1～3 min and the exposure area is relatively smaller;the illumination of 40 W is capable of curing the specimens without overcuring and yellowing in 10～30 s,and the exposure area is larger;The illumination of 60 W could cure the specimens to be yellow and hot within the shortest time due to the instant irradiated heat.However,by optimizing the illumination process and controlling the exposure area,it could be realized as well that the 60 W is adopted to cure a specimen in good quality within 5～30 s.As a result,the ultraviolet light with 40 W or 60 W is selected for solid propellant 3D-printing.

3.3 Effect of Al/ Pb2O3 content on UV-curable depth

The effect of Al powder content on the UV-curable depth of the propellant is shown in Table 4.Through the comparison of the results,as the Al content increases,the UV-curable depth gradually decreases.When the Al content is within 9%～11%,the UV-curable depth reaches the minimum 1 mm,which may lead to propellant slurry un-curing or poor curing;when the Al content is within 1%～2%,the UV-curable depth can be up to 12 mm,which may result in over-curing.Besides,the low content of Al would theoretically lower the energetic level of the propellant formulations and affect the thrust of the propellant grains.The deposited propellant layer is practically 1～2 mm in depth,thus,the Al content with 6%～8% is optimum for the propellant formulations,apparently.

Table 4 Effect of Al content on UV-curable depth

The effect of burning rate catalyst (PbO) content on UV-curable depth is similar to that of Al content (Table 5).The higher the burning rate catalyst (PbO) content,the smaller the UV-curable depth.It suggests that the higher content of PbOhas negative effect on the propellant photocuring.In consideration of the contribution of PbO,the 0.1% PbOcomtent is the relatively optimum selection.

Table 5 Effect of burning rate catalyst (Pb2O3) content on UV-curable depth

3.4 Effect of solids content on propellant slurry extrusion

The quality of propellant slurry extrusion is obviously subject to the solids content of the solid propellant formulations (Table 6).The higher solids content implies the higher viscosity from open literature.As the solids content is 90%,the propellant slurry is difficult to be extruded even under great pressure;while the solids content is less than 60%,the slurry would drop even under slight pressure,see Fig.7(b).Therefore,the optimum solids content is 70%～80% for the propellant formulations.

Table 6 Effect of solids content on propellant slurry extrusion

(a)80% solids content (b)60% solids contentFig.7 Effect of solids content on propellant slurry extrusion

The physical state of propellant slurry with 90% and 80% solids content is shown in Fig.8.The propellant slurry with 90% solids content (Fig.8(a)) is in a discontinuously discrete granular state.Overhigh solids content is the principal reason,leading to the fillers unable to be adhered fully.When the solids content decreases to 80% (Fig.8(b)),the slurry is in a uniformly continuously liquid state,owing to the sufficient liquid ingredients of the propellant formulations,which modifies the rheological property effectively.

The extrusion condition and viscosity of propellant slurry with 80% solids content are shown in Fig.9 and Table 7,respectively.The propellant slurry can be extruded smoothly from the 2 mm print-head,moreover,the extruded propellant slurry maintains shape fidelity without visible deformations.Besides,the slurry with 80% solids content maintains viscosity less than 956 Pa·s for 5 h,providing adequate pot time for 3D-printing.

(a)90% solids content (b)80% solids contentFig.8 Propellant slurry with 80% and 90% solids content.

Fig.9 Extrusion condition of propellant slurry with 80%solids content (2 mm print-head)

Table 7 Viscosity of propellant slurry with 80% solids content

3.5 Curing parameter

As shown in Table 8,the 3D-printed grains after cured are soft as the curing parameter=1.0,which might result from the less curing agent content;the 3D-printed grains after cured are harder as=1.2 because of the excessive curing agent content.Based on the testing results,the ideal curing parameterfor the 3D-printing propellant formulations is within 1.1～1.15,for the maximum tensile strength and maximum elongation meet the operating requiremants

Table 8 Effect of curing parameter on curing condition of 3D-printed propellant grain

The mechanical performance for the 3D-printed grains is described in Fig.10 and Table 9.With the same testing conditions,the averages of the maximum tensile strength and the maximum elongation for the 3D-printed grains are 0.299 MPa and 22.4%,respectively.

Fig.10 Tensile curves of 3D-printed grains

Table 9 Mechanical performance of 3D-printed grains

The fracture of dumbbell specimens is shown in Fig.11.The fracture surface of each printed layer is continuous,whereas the entire fracture surface is not continuous between layers,which appears sharp contrast with the entire continuous fracture surface of cast-cured grains.From this point of view,stress concentration appears primarily at the boundary between layers while the specimens are under tensile tests,indicating that the photocuring interval is shorter than the optimum and the vacuum condition is inadequate.

Fig.11 Fracture of dumbbell-shaped specimens

3.6 Porosity and density

The porosity images are shown in Fig.12.The porosity of 3D-printed grains is 6.8%,higher than that (2.98%) of the cast-cured grains.The main reason is probably the absence of vacuum pumping processing compared with the traditional cast-cured process.

The density of 3D-printed and cast-cured grain is 1.62 g/cmand 1.76 g/cm,respectively,which complements with the results of porosity.

Fig.12 The porosity image: 3D-printed grain(left);cast-cured grain(right)

4 Conclusions

Photocuring assisted 3D printing material extrusion technology is quite promising in the manufacturing field of composite solid propellant.A novel lab-made UV-curable polyether adhesive is added to modify the traditional HTPB solid propellant formulations,expecting to 3D-print propellant grains with not only an intricate geometry but also comparable mechanical performance.

To ensure the UV-curable polyether adhesive to cohere with the HTPB propellant,DSC is employed to characterize the decomposition temperature of the mixture (UV-curable polyether adhesive:AP/Al=1︰1).The results show that the compatibility of the adhesive and AP/Al is exactly right.To determine the UV-curable polyether HTPB propellant formulations,a series of tests including the selection of ultraviolet source,UV-curable depth test,propellant extrusion test and curing parameter test are conducted.Based on the designed UV-curable polyether HTPB propellant formulations,the grains with intricate geometry are 3D-printed successfully by an explosion-proof 3D printer.To evaluate the 3D-printed grain quality and feasibility of the designed solid propellant formulations,the tensile tests,porosity tests and density tests are performed.The corresponding tensile strength,maximum elongation,porosity and density are 0.299 MPa,22.4%,6.8% and 1.62 g/cm,respectively.

Optimizing of the formulations and 3D-printing process will be focused in future,aiming to increase the 3D-printed grain quality.Furthermore,the burning rate tests will be performed to characterize the combustion performance.