LOW-VISCOSITY EPOXY RESINS AND LOW VOC CURABLE FORMULATIONS THEREFROM

Abstract

The present invention provides curable moldable compositions comprising from 10 to 80 volume % of heat resistant fiber compositions, a two component resin mixture of (i) one or more epoxy resins, and (ii) a hardener comprising a combination of triethylenetetraamine (TETA) and from 3 to 15 wt. %, based on the weight of the TETA, of 2-phenylimidazole (2-PI). The compositions cure to provide composite articles having a short demold time of 90 s or less at 130 C. and 101 kPa and a high DSC glass transition temperature (Tg) of from 130 to 180 C. when cured at 130 C. for 90 seconds at a pressure of 101 kPa. The invention enables lightweight, heat resistant composite articles, such as for use in automotive applications.

Claims

1. A curable moldable composition comprising from 10 to 80 volume % of heat resistant fiber compositions, a two component resin mixture of (i) one or more epoxy resins, and (ii) a hardener comprising a combination of triethylenetetraamine (TETA) and from 3 to 15 wt. %, based on the weight of the TETA, of 2-phenylimidazole (2-PI), wherein the two component resin mixture has upon mixing a viscosity of from 10 to 100 mPa.Math.s at 130 C.

2. The curable moldable compositions as claimed in claim 1, wherein the heat resistant fiber composition is chosen carbon fiber, glass fiber, ceramic fiber, acrylonitrile fiber, aramid fiber, or their admixtures.

3. The curable moldable compositions as claimed in claim 1, wherein the (i) one or more epoxy resins is a bisphenol A or F diglycidyl ether epoxy resin.

4. The curable moldable compositions as claimed in claim 1, wherein the two component resin mixture of the curable moldable compositions has a DSC glass transition temperature (Tg) of from 130 to 180 C., when cured at 130 C. for 90 seconds at a pressure of 101 kPa.

5. The curable moldable compositions as claimed in claim 1, wherein the ratio of molar equivalents of the (i) one or more epoxy resins to the molar equivalents of amine hydrogens in the combination of TETA and 2-PI in the two component resin mixture ranges from 0.7:1 to 1.4:1.

6. The curable moldable compositions as claimed in claim 1, wherein the one or more heat resistant fiber compositions is chosen from a continuous fiber material, a non-woven fiber material, a mat or a stack of two or more mats, and a material comprising both continuous and discrete fibers.

7. The curable moldable compositions as claimed in claim 1, wherein the composition further comprises one or more impact modifiers, internal mold release agents, reactive diluents, coalescents, pigments, particulate fillers, extenders, tackifiers, antioxidants and wetting agents.

8. A composite article comprising a matrix of a cured two component resin mixture of (i) one or more epoxy resins, and (ii) a hardener comprising a combination of triethylenetetraamine (TETA) and from 3 to 15 wt. %, based on the weight of the TETA, of 2-phenylimidazole (2-PI), and, within the matrix, from 10 to 80 volume % of the total composite article, of a heat resistant fiber composition.

9. A composite article as claimed in claim 8, wherein the cured two component resin mixture has a DSC glass transition temperature (Tg) of from 130 to 180 C. when cured at 130 C. for 90 seconds at a pressure of 101 kPa.

10. A method of making a fiber reinforced resin matrix composite article comprising: forming a two component resin mixture of (i) one or more epoxy resins, and (ii) a hardener comprising a combination of triethylenetetraamine (TETA) and from 3 to 15 wt. %, based on the weight of the TETA, of 2-phenylimidazole (2-PI), wherein the two component resin mixture has upon mixing a viscosity of from 10 to 100 mPa.Math.s at 130 C.; wetting a heat resistant fiber composition comprising one or more heat resistant fibers in the amount of from 10 to 80 volume % of the total heat resistant fiber composition and the total two component resin mixture; and, curing the two component resin mixture at a temperature of from 60 to 200 C. and at a pressure of from 200 to 7500 kPa (2 to 100 bar) and for a time of from 15 to 300 s.

Description

EXAMPLES

[0079] The following examples are used to illustrate the present invention without limiting it to those examples. Unless otherwise indicated, all temperatures are ambient temperatures and all pressures are 1 atmosphere.

[0080] The following materials and chemicals were used in the Examples that follow:

[0081] Epoxy Resin 1: Liquid epoxy resins (The Dow Chemical Co., Midland, Mich. (Dow)), each of which is a digycidyl ether of bisphenol A, EEW 175 to 181 g/eq and less than 1 wt. % of monohydrolyzed resin;

[0082] Epoxy Resin 2: A blend of Epoxy Resin 1 (60 wt. %), and [D.E.N. Dow]epoxy novolac resin, EEW 175-181 g/eq (40 wt. %); Epoxy Resin 3: A blend of Epoxy novolac resin, EEW 176 to 181 g/eq (15 wt. %), a diglycidyl ether of bisphenol A, having an epoxy equivalent weight of from 182-192 (15%), and 3,4-Epoxycyclohexyl methyl-3,4-epoxycyclohexanecarboxylate (CAS: 2386-87-0) EEW: 131-143 (70%);

[0083] DETA: diethylene triamine, ANEW 20.6 g/eq;

[0084] TETA: triethylenetetraamine, AHEW 24.4 g/eq;

[0085] 2-PI: 2-phenylimidazole, CAS: 670-96-2;

[0086] Hardener 2: A mixture of 93 wt. % of TETA and 7 wt. % of 2-PI;

[0087] Hardener 3: A mixture of 1,2-Diaminocyclohexane (CAS#694-83-7) 95 wt. % and 2-PI 5 wt. %;

[0088] Hardener 4: A mixture of Nadic METH E methyl anhydride (Polynt S.p.A., Bergamo IT) (85.01 wt. %), core shell rubber PARALOID EXL 2650 A (11.41 wt. %), HYCAT 3000s (Dimension Technology Chemical Systems, Inc., Fair Oaks, Calif.) trivalent chromium (III) carboxylate complex containing <10 wt. % phenol and <10 wt. % benzyldimethylamine (1.79 wt. %), 1 methyl imidazole (1.79 wt. %).

[0089] Test Methods:

[0090] Dynamic Differential Scanning Calorimetry (DSC):

[0091] Dynamic DSC was used to determine the reported Tg of the indicated resins and formulations, wherein each indicated material was heated from 25 to 200 C. on a heating ramp of 20 C./min, then kept isothermal at 200 C. for three minutes, then cooled on a ramp of 20 C./min down to 25 C. and was kept isothermal at 25 C. for three minutes, and was then heated to 200 C. a second time while DSC was performed using a heating ramp of 20 C./min, followed by keeping the material isothermal at 200 C. for 3 minutes, and cooling on a ramp of 20 C./min to 25 C. Tg onset and Tg midpoint were determined, respectively, as the onset of the plateau on the resulting DSC curve and the inflection point on the resulting DSC curve.

[0092] Gel Time and Demold Time:

[0093] The indicated epoxy resin (at approximately 40 C.) and hardener composition (at approximately 25 C.) were brought together in the indicated ratio and mixed for 30 seconds. The resulting mixture was poured onto a hot plate preheated to 130 C. to form a disk of the mixture on the surface of the plate, at which point a timer was started at 0. Time was measured from the point at which the mixture contacts the hot plate surface. The hot plate was maintained at 130 C. as the mixture cured. Periodically, a pallet knife was pulled through the liquid disk to displace the mixture. The gel time (GT) was recorded as the time after which the mixture would no longer flow into the scored line. Demold time (DMT) was recorded as the time after pouring at which the disk can be removed from the hot plate surface as a solid, using the pallet knife. The Gel Time and Demold Time reported in Table 1, below, represent the value taken from one trial for each indicated material.

[0094] Composite 90 Tensile Modulus:

[0095] This property was measured according to standard ISO 527-4 (International Organization for Standardization, Geneva, C H, 1997) using a unidirectional (UD) fabric).

[0096] Composite 90 Tensile Strength:

[0097] This property was measured according to standard ISO 527-4 (1997) with UD fabric.

TABLE-US-00001 TABLE 1 Two Component Resin Mixtures and Their Performance Gel Demold Tg Tg Mix Ratio Time Time onset midpt Example Resin Hardener Stoichiometry (s) (s) ( C.) ( C.) Comp 1 Epoxy TETA 100:13.7 1:1 37 >300 102 112 Resin 1 Comp 2 Epoxy TETA 100:15.1 .sup.1:1.1 35 >300 96 111 Resin 1 Comp 3 Epoxy TETA 100:13.9 1:1 28 180 124 133 Resin 2 1 Epoxy Hardener 2 100:14.7 1:1 34 90 134 141 Resin 1 2 Epoxy Hardener 2 100:16.2 .sup.1:1.1 32 90 130 140 Resin 1 3 Epoxy Hardener 2 100:14.9 1:1 26 60 142 156 Resin 2

[0098] Comparative Examples 1 and 2 demonstrate that use of the TETA-E hardener with the epoxy resin 1 obtain a low Tg with long demold time. However, good latent behavior is shown in terms of the gel time. This latent behavior is important to fill molds for large complex parts with high carbon fiber volume fractions of 50% or more and is a key feature to maintain in any new formulation. Inventive Examples 1 and 2 including 2-phenylimidazole demonstrate the ability to maintain gel time (positive latent behavior) while significantly increasing the glass transition temperature (by up to 30 C.) and reducing the demolding time (by over 210 seconds) under the cure conditions. Using epoxy resin 2, comprising a bisphenol A resin combined with a novolac epoxy resin to increase the DSC Tg of the resin, inventive Example 3 including 2-phenylimidazole gave increased DSC Tg and a dramatically faster demold time at 130 C. and 101 kPa without sacrificing the open time of comparative Example 3.

[0099] A composite panel in Example 4 was produced via a hand-mixed, hand-poured wet compression process. In this process, the two component resin mixture of a resin component (50 C.) and a hardener component (20 to 25 C.) was mixed via a drill mixer for a period of 30 seconds at the required ratio before being poured directly on top of a fiber preform comprised of 6 layers of unidirectional carbon fiber fabric that resided in an open mold at 130 C. During the pouring on to the carbon fiber preform, the two component resin mixture was laid down in bands from top to bottom of the preform. Once the two component resin mixture was fully filled into the mold, the mold was closed to allow impregnation of the two component resin mixture into the carbon fiber preform and for the part to achieve full cure. The panel was cured for 180 seconds.

[0100] In Comparative Examples 4 and 5, composite panels were formed in a KraussMaffei high pressure 2-component epoxy RTM machine (KraussMaffei Technologies GmbH) operating on a 120-ton press equipped with a thermostated steel mold capable of creating rectangular plaques having dimensions 5002702 mm. The temperature of the components in the epoxy resin component was set to 80 C. and to 50 C. for the hardener component; catalysts, if any, were pre-blended with the hardener. The mold temperature was set to 140 C. in Comparative Example 4 and 160 C. in Comparative Example 5; injection of the two component resin mixture was performed through an injection hole located in the top part of the mold after mold closure and subsequent evacuation to 0.02 bars. The part in Comparative Examples 4 and 5 was molded 240 seconds.

[0101] Glass transition temperatures in Table 2, below, were tested by removing a circular piece (5 mm diam2 mm thick) of the molded composite and placing that in the calorimeter (DSC).

TABLE-US-00002 TABLE 2 More Results Comparative Example 5 (from U.S. Comparative 8742018B2 to Units Example 4 Reynolds et al.) Example 4 T.sub.g mid-point C. 195 210 135 (DSC) Composite 90 MPa 8050 6800 8300 Tensile Modulus Composite 90 MPa 45 48 56 Tensile Strength Process Mold C. 140 160 130 Temperature Gel Time (hot s 80 60 32 plate test) Demold Time s >240 240 180 (actual) Post cure Yes No No required Fabrication RTM RTM LCM Method

[0102] In Table 2, above, the indicated materials were tested as indicated above. Where indicated, a post cure was performed in an oven. In Comparative Example 4, epoxy resin 2 was used and the Hardener 3 was used, wherein the epoxy resins and the hardener were mixed at a weight proportion of 100 (resin): 17 (Hardener). To achieve a suitable final cure performance an isothermal post cure was carried out for 30 mins at @ 200 C. In Comparative Example 5, Epoxy resin 3 and Hardener 4 was used in a weight ratio of 100 (resin): 136 (Hardener). In Inventive Example 4, Epoxy Resin 1 and Hardener 2 were used at a mix ratio of 100 (resin): 16.2 (Hardener).

[0103] Not shown in any table, the two component resin mixture Inventive Example 4 cured in 90 s or less to give a cured resin having a DSC Tg of above 135 C. without the use of a hardener component comprising a cycloaliphatic compound and without any post cure. This demonstrates the capability of the inventive compositions to enable mass production of carbon fiber composites with higher build volumes via the RTM or LCM techniques.

[0104] As shown in Table 2, above, in Example 4 final composite performance can be gained out of tool with no requirement for additional process steps such as post curing. Further, as shown by Tensile Modulus and Tensile Strength results, the material produced in inventive Example 4 does not require a toughening agent to perform like that of Comparative Example 5. In addition, all of the Comparatives in Table 2 above, were formed in a vacuum using a resin transfer process which gives them the expected advantage of improved resin flow, transfer and filling the mold. The inventive two component resin mixture in Example 4 gave a surprisingly short gel time and, in a molded part, exhibited a dramatic improvement in tensile strength after a shorter mold time.