THERMOSETTING MATERIAL FOR USE IN ADDITIVE MANUFACTURING

Abstract

The present invention relates to a thermosetting material for use in additive manufacturing, the material comprising at least one thermosetting resin and at least two curing compounds different from said thermosetting resin that are able to cure this/these thermosetting resin(s), wherein at least one curing compound is provided for curing during the additive manufacturing process and at least one curing compound is provided for curing during a post-curing step. The invention furthermore relates to a method of producing a cured 3D thermoset object comprising at least the steps of subjecting the material according to the present invention to an additive manufacturing process, obtaining a partially cured 3D thermoset object and subsequently subjecting the partially cured 3D thermoset object to a post-curing process to further cure the 3D thermoset object Additionally, the invention relates to the use of the material in an SLS, FFF, CBAM, FGF or powder bed additive manufacturing process.

Claims

1. Thermosetting material for use in additive manufacturing, the material comprising at least one thermosetting resin and at least two curing compounds different from said thermosetting resin able to cure this/these thermosetting resin(s), wherein at least one curing compound is provided for curing during the additive manufacturing process and at least one curing compound is provided for curing during a post-curing step.

2. Thermosetting material according to claim 1, characterized in that it comprises at least two thermosetting resins.

3. Material according to claim 1, characterized in that the curing compound provided for curing during the additive manufacturing process is present in the material in a sub-stoichiometric amount with regard to the reactive groups present in the thermosetting material.

4. Material according to claim 1, characterized in that the or at least one of the curing compound(s) provided for curing during a post-curing step has an activation temperature above the temperature used in the additive manufacturing process, as determined by means of DSC analysis according to the method given in the description.

5. Material according to claim 1, characterized in that at least one of the curing compounds has an activation temperature of above 80° C., preferably of above 100° C., as determined by means of DSC analysis according to the method given in the description.

6. Material according to claim 1, characterized in that the difference regarding activation temperature of at least two curing compounds is at least 10° C., preferably at least 20° C., with the activation temperature being determined by means of DSC analysis according to the method given in the description.

7. Material according to claim 1, characterized in that it comprises at least a third curing compound having a temperature difference regarding activation temperature from the first curing compound of at least 20° C., preferably at least 40° C., more preferably at least 60° C., with the activation temperature being determined by means of DSC analysis according to the method given in the description.

8. Material according to claim 1, characterized in that it comprises at least one curing compound having an activation temperature of above 70° C., at least one curing compound having an activation temperature of above 100° C., and at least one curing compound having an activation temperature of above 140° C., with the activation temperature being determined by means of DSC analysis according to the method given in the description.

9. Material according to claim 1, characterized in that at least one curing compound is provided for curing during a post-curing step by radiation.

10. Material according to claim 1, characterized in that at least one curing compound is provided for a thermal post-curing step.

11. Material according to claim 1, characterized in that it comprises at least three curing compounds.

12. Material according to claim 1 in form of powder, granulate and/or filament.

13. A method of producing a cured 3D thermoset object comprising at least the following steps: a. subjecting the material according to claim 1 to an additive manufacturing process; b. obtaining a partially cured 3D thermoset object and; c. subjecting the partially cured 3D thermoset object to a post-curing process, preferably comprising a heat treatment, to further cure the 3D thermoset object.

14. Use of a material according to claim 1 in a selective laser sintering (SLS), fused filament fabrication (FFF), composite-based additive manufacturing (CBAM), fused granular fabrication (FGF) or powder bed additive manufacturing process and more preferably a selective laser sintering process.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0140] FIG. 1 shows typical DSC curves of a material according to the present invention. In the first run (grey line), the curing can clearly be observed by the presence of the exothermic reaction starting at above 100° C., which is attributed to a curing reaction caused by the curing compound that is provided for the additive manufacturing process; the peak of this first curing reaction is at about 140° C. The exact peak temperature could be obtained by the analysis software. The start of the second curing reaction, which is caused by the curing compound, that is provided for the post-curing step, is at above 170° C. The maximum reaction rate and thus the maximum formation of exothermic reaction heat is reached at the peak temperature of 218° C. At such temperature, the object/material can be fully cured in the course of a post-curing step. In the second and third DSC-run (black lines), no further exothermic reaction is observed, indicating that the material is fully cured. The endothermic peak slightly below 100° C. indicates a melting of a (semi)crystalline compound of the material. The Tg of this material is 44° C.

[0141] FIG. 2 presents DSC curves of a material with only one curing compound, which is reactive at about 100° C. and above, that is provided for curing during the additive manufacturing process. Such material is significantly more sensitive towards undesired pre-reactions during storage or transport because of the high reactivity and high content of the curing compound as compared to an inventive material. Also, such a material usually cannot be reused and suffers strong thermal bleeding.

[0142] FIG. 3 shows DSC curves of a material that only contains one curing compound, which is reactive at about 160° C. and above, that is provided for curing during the post-curing step. The activation temperature (onset temperature) is higher than 160° C. in this case, namely 186° C. The peak temperature is 197° C. Such material does not show any or only insignificant curing during the additive manufacturing step (the energy of such process is chosen such that only a more reactive curing compound that is provided for the additive manufacturing step reacts to a significant extent; e.g.: a compound as used in FIG. 2) and thus may not be removed from the printer without being damaged or may not even be dimensionally stable in the course of the post-curing step.

[0143] FIG. 4 shows typical graphs obtained from the viscosity measurement of a thermosetting material according to the present invention. The black line with the lowest minimum shows the viscosity (|η*|). The temperature increase is shown by the grey line. As can be seen, the viscosity first decreases, reaches the minimum and then starts to increase as a result of the curing of the material. It can be clearly seen, that the minimum of the viscosity curve is formed by an overlap of two distinct reactions, which are caused by two curing compounds, one of which is provided for the additive manufacturing process and the other one for the post-curing step. The reaction of the curing compound provided for the additive manufacturing process occurs already at lower temperature and decreases the negative slope of the viscosity curve. Such reaction provides dimensional stability to an object during the additive manufacturing process. Nevertheless, the viscosity still slowly decreases to reach a minimum due to softening and/or melting of the compounds of the material upon further heating. The following rapid increase of viscosity reflects the further crosslinking. Also, from FIG. 4. it can be seen, that the material still possesses good flowability, even if the first curing compound as provided for the additive manufacturing process already reacted to a large extent. This flowability enables sufficient flow of the material in the course of the post-curing step, which results in better layer coalescence and thus in improved isotropy of the obtained objects, however, without loss of shape or dimensional accuracy due to the partial curing of the more reactive curing compound.

[0144] FIG. 5 shows typical graphs obtained from the viscosity (|η*|) measurement of a thermosetting material comprising only one curing compound. It can be seen that only kind of curing reaction occurs. Since only one curing compound is used which has a reactivity starting at 80° C., it is difficult to control the reactivity of the material, upon storage and also in the course of the additive manufacturing process, resulting in undesired pre-reactions of the material upon storage and of excess material during the additive manufacturing process. The material reacts much faster with increasing temperature than the material according to the present invention, resulting in a higher minimum viscosity, causing deteriorated layer coalescence and thus anisotropic properties of the objects. Also, the excess material may not be reused.

[0145] FIG. 6 shows an illustrative example for the determination of the activation temperature of a thermosetting resin with a curing compound by means of a DSC measurement According to the definition and method given within the present application, the activation temperature, corresponds to the onset point of the exothermic curing peak, being 141° C.

EXAMPLES

Inventive Example 1

[0146] The material was composed of D.E.R. 642U (Epoxy Resin; 50.0 wt %, EEW=500-560 q/eq; viscosity=1.9-3.3 Pas (150° C.), softening point=89-97° C., Dow Chemical Company, US), Epiclon HP-4710 (naphthalene based epoxy resin, 6.0 wt %, softening point=96° C., EEW=171 g/eq, viscosity=1.0 Pas (150° C.), DIC Corp., Japan); Sirales PE 5900 ((semi)crystalline, carboxy functional polyester, 8% wt %, AV=28-36 mgKOH/g, viscosity=1.3 Pas (150° C.), Mp=110° C., melting range of 105-120° C., Sir Industriale); Kukdo KR-102 (rubber modified Bisphenol-A-type epoxy resin; 10.0 wt %, EEW=1100-1300, Kukdo Chemical Co, Ltd., South Korea); Aradur 835 CH (aliphatic polyamine curing compound, 2.0 wt %, reactive at >70° C. Huntsman, US); CUREZOL 2MZ (imidazole curing compound, 1.0 wt %, reactive at >100° C., SHIKOKU); DYHARD® 100S (Dicyandiamide curing compound, 1.5 wt %, reactive at >150° C. Alz Chem, Germany); Aktifit AM (filler, 10.0 wt %, Hoffmann Mineral GmbH, Germany), Benzoin=Hydroxybenzylphenyl-Ketone (degassing additive, 1.0 wt %, Harke Chemicals GmbH, Germany); Resiflow PL200 (flow additive, 2.5 wt %, Estron Chemical Inc., US), TI-Select TS6200 (pigment additive, 8 wt %, DuPont Titanium Technologies, US). The total quantities of each compound were chosen such that 40 kg of powder was obtained. All compounds were premixed in a high-speed mixer (Thermo PRISM Pilot 3, Thermo Fisher Scientific, US) for 1 min at 25° C. with a rotor speed of 1000 rpm and then extruded in a twin-screw ZSK-18 extruder (Coperion, Germany) at a screw speed of 400 rpm with a temperature gradient from 40 to 100° C. and a cooling device for the feeding area was used. The mixture obtained was then cooled down, granulated and fine ground to obtain a powder material having a D50 of less than 80 μm. The Tg of the material was 60° C.

Inventive Example 2

[0147] D.E.R.™ 6510-HT (epoxy resin, 41.5 wt %, EEW=400-450 q/eq, viscosity=7.5-9.5 Pas (150° C.), softening point=105-114° C.), Dow Chemical Company); Epiclon® HP-4710 (naphtalene based epoxy resin, 12.0 wt %, softening point=96° C., EEW=171 g/eq, viscosity=1.0 Pas (150° C.), DIC Corp.); HyPox® RK84L (CTBN-modified epoxy resin, 10.0 wt %, softening point is not measurable, EEW=1250-1500 g/eq, viscosity=300-500 Pas (100° C.), Huntsman), Aradur 835 CH (aliphatic polyamine curing compound; 1.5 wt %, reactive at >70° C., Huntsman), CUREZOL 2MZ (imidazole curing compound, 1.0 wt %, reactive at >100° C. SHIKOKU); DYHARD® 100S (Dicyandiamide curing compound, 0.5 wt %, reactive at >150° C. Alz Chem); Aktifit AM (filler, 9 wt %, Hoffmann Mineral GmbH), Benzoin (Hydroxybenzylphenyl-Ketone degassing additive, 0.5 wt %, Harke Chemicals GmbH); Resiflow PL200 (flow additive, 2.0 wt %, Estron Chemical Inc.); TI-Select TS6200 (pigment filler, 10 wt %, DuPont Titanium Technologies), Exolit® OP1230 (organic phosphinate flame retardant filler, 12 wt %, Clariant Ltd., Switzerland). The material was produced in full analogy to Example 1. The Tg of the material was 63° C.

Inventive Example 3

[0148] D.E.R.™ 6510-HT (epoxy resin, 41.5 wt %, EEW=400-450 q/eq, viscosity=7.5-9.5 Pas (150° C.), softening point=105-114° C.), Dow Chemical Company); Epiclon® HP-4710 (naphtalene based epoxy resin, 12.0 wt %, softening point=96° C., EEW=171 g/eq, viscosity=1.0 Pas (150° C.), DIC Corp.); HyPox® RK84L (CTBN-modified epoxy resin, 10.0 wt %, softening point is not measurable, EEW=1250-1500 g/eq, viscosity=300-500 Pas (100° C.), Huntsman), Aradur 835 CH (aliphatic polyamine curing compound; 1.5 wt %, reactive at >70° C., Huntsman), Epikure P-108 (accelerated dicyandiamide curing agent, 1.5 wt %, Hexion Chemicals, US); Aktifit AM (filler, 9 wt %, Hoffmann Mineral GmbH), Benzoin (Hydroxybenzylphenyl-Ketone degassing additive, 0.5 wt %, Harke Chemicals GmbH); Resiflow PL200 (flow additive, 2.0 wt %, Estron Chemical Inc.); TI-Select TS6200 (pigment filler, 10 wt %, DuPont Titanium Technologies), Exolit® OP1230 (organic phosphinate flame retardant filler, 12 wt %, Clariant Ltd.). The material was produced in full analogy to Example 1. The Tg of the material was 65° C.

Inventive Example 4

[0149] D.E.R.™ 6510-HT (epoxy resin, 41.5 wt %, EEW=400-450 q/eq, viscosity=7.5-9.5 Pas (150° C.), softening point=105-114° C.), Dow Chemical Company); Epiclon® HP-4710 (naphtalene based epoxy resin, 12.0 wt %, softening point=96° C., EEW=171 g/eq, viscosity=1.0 Pas (150° C.), DIC Corp.); HyPox® RK84L (CTBN-modified epoxy resin, 10.0 wt %, softening point is not measurable, EEW=1250-1500 g/eq, viscosity=300-500 Pas (100° C.), Huntsman), Aradur 835 CH (aliphatic polyamine curing compound; 1.5 wt %, reactive at >70° C., Huntsman), DYHARD® MI-C(imidazole curing compound, 0.5 wt %, reactive at >120° C. AlzChem, Germany); DYHARD® 100S (Dicyandiamide curing compound, 1.0 wt %, reactive at >150° C. Alz Chem); Aktifit AM (filler, 9 wt %, Hoffmann Mineral GmbH), Benzoin (Hydroxybenzylphenyl-Ketone degassing additive, 0.5 wt %, Harke Chemicals GmbH); Resiflow PL200 (flow additive, 2.0 wt %, Estron Chemical Inc.); TI-Select TS6200 (pigment filler, 10 wt %, DuPont Titanium Technologies), Exolit® OP1230 (organic phosphinate flame retardant filler, 12 wt %, Clariant Ltd.). The material was produced in full analogy to Example 1. The Tg of the material was 64° C.

Inventive Example 5

[0150] D.E.R. 642U (Epoxy Resin; 50.0 wt %, EEW=500-560 q/eq; viscosity=1.9-3.3 Pas (150° C.), softening point=89-97° C., Dow Chemical Company), Epiclon HP-4710 (naphthalene based epoxy resin, 6.0 wt %, softening point=96° C., EEW=171 g/eq, viscosity=1.0 Pas (150° C.), DIC Corp.); Sirales PE 5900 ((semi)crystalline, carboxy functional polyester, 8% wt %, AV=28-36 mgKOH/g, viscosity=1.3 Pas (150° C.), Mp=110° C., melting range of 105-120° C., Sir Industriale, Italy); Kukdo KR-102 (rubber modified Bisphenol-A-type epoxy resin; 10.0 wt %, EEW=1100-1300, Kukdo Chemical Co, Ltd.); Aradur 835 CH (aliphatic polyamine curing compound, 3.0 wt %, reactive at >70° C. Huntsman); DYHARD® 100S (Dicyandiamide curing compound, 1.5 wt %, reactive at >150° C. AlzChem); Aktifit AM (filler, 10.0 wt. %, Hoffmann Mineral GmbH), Benzoin=Hydroxybenzylphenyl-Ketone (degassing additive, 1.0 wt %, Harke Chemicals GmbH); Resiflow PL200 (flow additive, 2.5 wt %, Estron Chemical Inc.), TI-Select TS6200 (pigment additive, 8 wt %, DuPont Titanium Technologies). The material was produced in full analogy to Example 1. The Tg of the material was 64° C.

Inventive Example 6

[0151] D.E.R.™ 6510-HT (epoxy resin, 32.0 wt %, EEW=400-450 q/eq, viscosity=7.5-9.5 Pas (150° C.), softening point=105 −114° C.), Dow Chemical Company); Crylcoat® 1514-2 (carboxylated polyester resin, 32.0 wt %, Tg=55° C., Acid Value=68-74 mg KOH/g, viscosity=7.5-11 Pas (175° C.), Annex, Austria); Aradur 835 CH (aliphatic polyamine curing compound; 1.5 wt %, reactive at >70° C., Huntsman), DYHARD® 100S (Dicyandiamide curing compound, 1.0 wt %, reactive at >150° C. Alz Chem); Aktifit AM (filler, 9 wt %, Hoffmann Mineral GmbH), Benzoin (Hydroxybenzylphenyl-Ketone degassing additive, 0.5 wt %, Harke Chemicals GmbH); Resiflow PL200 (flow additive, 2.0 wt %, Estron Chemical Inc.); TI-Select TS6200 (pigment filler, 10 wt %, DuPont Titanium Technologies), Exolit® OP1230 (organic phosphinate flame retardant filler, 12 wt %, Clariant Ltd.). The material was produced in full analogy to Example 1. The Tg of the material was 59° C.

Comparative Example 1

[0152] D.E.R 642U (Epoxy Resin; 48.0 wt %, EEW=500-560 q/eq; viscosity=1.9-3.3 Pas (150° C.), softening point=89-97° C., Dow Chemical Company); Epiclon® HP-4710 (naphtalene based epoxy resin, 12.0 wt %, softening point=96° C., EEW=171 g/eq, viscosity=1.0 Pas (150° C.), DIC Corp.); HyPox® RK84L (CTBN-modified epoxy resin, 10.0 wt %, softening point is not measurable, EEW=1250-1500 g/eq, viscosity=300-500 Pas (100° C.), Huntsman); 2-Ethylimidazole (curing compound, 0.5 wt %, reactive at >100° C. Donauchem GmbH); Epikure P-108 (accelerated Dicyandiamide curing compound, 1 wt %, reactive at >100° C. Hexion Chemicals); Lanco TF 1778 (additive, 0.5 wt %, Lubizol); Ti-select (pigment additive, 13.0 wt %, DuPont); Tremin VP 939-600 EST (wollastonite filler, 15 wt %, HPF, Quarzwerke, Germany). The material was produced in full analogy to Example 1. The Tg of the material was 53 C.°

[0153] Printing Tests

[0154] In order to assess the suitability of the material, according to the present invention to be used in an additive manufacturing process, 3D printing tests were performed. An SLS 3D printing process was chosen, as the materials according to the present invention are particularly suitable to be provided in powder form. However, the materials according to the present invention may also be used for various 3D printing processes other than SLS.

[0155] [SLS printing process] A PRODWAYS ProMaker P1000 SLS 3D Printer (Prodways, France) was used for all the SLS 3D printing tests. Tensile bars according to DIN EN ISO 527-1:2019 were printed. The parameter settings of the SLS 3D printer are listed in Table 1.

TABLE-US-00002 TABLE 1 SLS 3D printer parameter settings for the printing tests laser hatch layer powder bed feeder power distance scan speed thickness temperature temperature [W] [mm] [mm/sec] [mm] [° C.] [° C.] 16 0.16 3500 0.10 65 50

[0156] After the 3D printing process, the objects (partially cured 3D thermoset objects) were unpacked from the powder bed by hand and sandblasted by using a Guyson Formula 1200 blast cabinet (Guyson Corp., US).

[0157] [Post-Curing]

[0158] The printed objects were post-cured in a convection oven (Thermo Fisher Scientific (US), Heraeus OVEN 199L) using the following temperature program: start at 25° C.—the printed object is put in the oven; heating 1° C./min; holding for 30 minutes at 100° C.; heating: 1° C./min; holding 15 minutes at 150° C.; heating 1° C./min and further holding 15 minutes at 200° C. Then the post-cured objects (fully cured 3D thermoset objects) were cooled to ambient temperature (25° C.). Finally, full curing of the 3D thermoset objects was confirmed by DSC (the shift in Tg was below 0.5° C. in two consecutive DSC measurements according to the method as described above).

[0159] [Gel Time Measurements]

[0160] Material of inventive example 1 and comparative example 1 were stored for 8 hours at 65° C. in an oven and the gel time of the materials were measured at 150° C. before and after storage. The results are shown in Table 2.

TABLE-US-00003 TABLE 2 Gel time measurement results of thermally treated materials Inventive Comparative Example 1 example 1 Gel time before storage 215 s at 194 s at at 65° C. 150° C. 150° C. Gel time after 8 hours 195 s at 20 s at of storage at 65° C. 150° C. 150° C.

[0161] The gel time of the material of inventive example 1 exhibits higher storage stability at 65° C. compared to the material of comparative example 1 as indicated by the difference in gel time reduction. A reduction of gel time indicates pre-reactions (curing) of the material.

[0162] This measurement simulates the temperature treatment of the material in the powder bed of a SLS printer and the results clearly show that the reusability of non-printed material (=e.g. excess material of the powder bed or from the overflow containers) is largely improved by the present invention.

[0163] To further determine the storage stability of the materials, samples were stored at 25° C. over 5 days and their gel times were determined at 150° C. before and after storage. The material of inventive example 1 shows no significant change in gel time after 5 days and the deviation is within the measurement error range of ±3 seconds. The gel time of the material of comparative example 1 shows a significant decrease of above 50 seconds within 5 days. The material according to the invention significantly increases the storage stability of the thermosetting materials. The results of the gel time measurements are listed in Table 3.

TABLE-US-00004 TABLE 3 Storage stability of inventive example 1 and comparative example 1 at 25° C. Storage time Inventive Comparative at 25° C. Example 1 Example 1 Before 215 s at 150° C. 200 s at 150° C. storage 1 day  215 s at 150° C. 194 s at 150° C. 2 days 213 s at 150° C. 189 s at 150° C. 3 days 214 s at 150° C. 169 s at 150° C. 4 days 215 s at 150° C. 150 s at 150° C. 5 days 214 s at 150° C. 142 s at 150° C.

[0164] Characterization of the Fully Cured 3D Thermoset Objects

[0165] [Mechanical properties] The mechanical characterization of the printed and fully post-cured objects was carried out using a commercially available tensile test (DIN EN ISO 527-1:2019) facility (Shimadzu AGS-10kNXD series). A clamping length of 115 mm and a crosshead speed of 5 mm/min were applied. The following Table 4 shows the determined mechanical properties of fully cured tensile bars. Based on the tensile test, the tensile modulus, stress at break and the strain at break were determined according to DIN EN ISO 527-1: 2019.

TABLE-US-00005 TABLE 4 Mechanical properties of the fully cured tensile bars Tensile modulus Stress at Strain at [GPa] break [MPa] break [%] Example 1 2.6 48.7 2.7 Example 2 2.5 46.0 2.4 Comparative Example 1 3.0 34.9 1.3