Storage stable epoxy prepregs from dicyandiamide solutions and methods for making the same

Abstract

The present invention provides thermosetting resin pre-impregnated or infused fiber materials or prepregs comprising a fiber material of a heat resistant fiber, such as a continuous fiber material or a discontinuous chopped fiber mat, infused with a thermosetting resin mixture comprising (i) at least one liquid epoxy resin; (ii) at least one epoxy novolac resin, (iii) dicyandiamide and (iv) and an adduct of a cycloaliphatic amine and a liquid epoxy resin, wherein the dicyandiamide is dissolved in the adduct of a cycloaliphatic amine and a liquid epoxy resin. The prepreg or fiber material has a shelf life of at least 30 days at ambient temperature and pressure before its Initial Tg (DSC) rises above 40° C.

Claims

1. A thermosetting resin pre-impregnated fiber material or prepreg comprising a fiber material of a heat resistant fiber infused with a thermosetting resin mixture comprising (i) at least one liquid epoxy resin, (ii) at least one epoxy novolac resin, (iii) dicyandiamide, and (iv) an adduct of a cycloaliphatic amine and a liquid epoxy resin, wherein the dicyandiamide is dissolved in the adduct of a cycloaliphatic amine and a liquid epoxy resin wherein the (iv) adduct of a cycloaliphatic amine and a liquid epoxy resin comprises the adduct of the liquid epoxy resin and a mixture of 4-methylcyclohexane-1,3-diamine and 2-methylcyclohexane-1,3-diamine (MDACH).

2. The thermosetting resin pre-impregnated fiber material or prepreg as claimed in claim 1, wherein the fiber material comprises a continuous fiber, a woven mat, or chopped or recycled fibers.

3. The thermosetting resin pre-impregnated fiber material or prepreg as claimed in claim 1, wherein the fiber material or prepreg has an initial (DSC) glass transition temperature (Initial Tg) of 40° C. or below, and, further wherein the ratio of amine hydrogen equivalents in the (iv) adduct of a cycloaliphatic amine and a liquid epoxy resin to the epoxy group equivalents in the (i) at least one epoxy resin and (ii) at least one novolac resin is from 0.25:1 to 0.45:1, and, yet further wherein, the ratio of amine hydrogen equivalents of the (iii) dicyandiamide to the total epoxy group equivalents in the (i) at least one epoxy resin and the (ii) at least one epoxy novolac resin ranges from 0.20:1 to 0.75:1.

4. The thermosetting resin pre-impregnated fiber material or prepreg as claimed in claim 1, wherein the fiber material or prepreg has a shelf life of at least 30 days at ambient temperature and pressure before its Initial Tg rises above 40° C.

5. The thermosetting resin pre-impregnated fiber material or prepreg as claimed in claim 1, further comprising one or more catalysts in amounts of from 1 to 8 phr, and wherein alter curing for 3 minutes at 150° C., the resulting material has a cured Tg of 150° C. or greater.

6. The thermosetting resin pre-impregnated fiber material or prepreg as claimed in claim 5, wherein the one or more catalyst comprises an alkylaryl or phenyl substituted urea.

7. The thermosetting resin pre-impregnated fiber material or prepreg as claimed in claim 1, wherein the continuous fiber material comprises a heat resistant fiber chosen from carbon fiber, glass fiber, ceramic fiber, acrylonitrile fibers, aramid fibers, or their admixtures.

8. A method for making a thermosetting resin pre-impregnated fiber material or prepreg comprising: infusing at ambient temperature a fiber material of a heat resistant fiber with a thermosetting resin mixture comprising (i) at least one liquid epoxy resin, (ii) at least one epoxy novolac resin, and a solution of (iii) dicyandiamide in (iv) an adduct of a cycloaliphatic amine and a liquid epoxy resin, the thermosetting resin mixture having a differential scanning calorimetry (DSC) initial glass transition temperature (Initial Tg) of 40° C. or below, wherein the ratio of amine hydrogen equivalents in the adduct of a cycloaliphatic amine and a liquid epoxy resin to the epoxy group equivalents in the (i) at least one epoxy resin and the (ii) at least one epoxy novolac resin ranges from 0.25:1 to 0.45:1; yet further wherein, the ratio of amine hydrogen equivalents of the (iii) dicyandiamide to the total epoxy group equivalents in the (i) at least one epoxy resin and the (ii) at least one epoxy novolac resin ranges from 0.20:1 to 0.75:1, and, then, B-staging the resulting material by heating it at a temperature of from 80 to 110° C. for a period of 30 seconds to 4 minutes to advance the Initial Tg of the thermosetting resin pre-impregnated fiber material or prepreg to a temperature of 40° C. or below.

Description

EXAMPLES

(1) 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 101 kPa (1 atmosphere).

(2) The following materials and chemicals were used in the Examples that follow:

(3) Baxxodur™ ECX 210: A mixture of 4-methylcyclohexane-1,3-diamine and 2-methylcyclohexane-1,3-diamine (MDACH), AHEW 32 g, BASF, Florham Park, N.J.;

(4) Carbon fiber (12K fibers/tow, A42-D012, DowAksa, Turkey); and

(5) NCF: Non Crimp Fabric of carbon fiber (590 g/m.sup.2)

(6) TABLE-US-00001 TABLE 1 Example 1 Formulation Name Description Epoxy Resin 1 Liquid epoxy resin of a diglycidyl ether of Bis-phenol A (EEW 176-182 g) Epoxy Resin 2 Liquid epoxy Novolac resin (EEW 175-181 g) Technicure ™ nano Dicy Dicyandiamide (A&C Catalysts Linden, NJ, AHEW 21 g) MDACH - Epoxy Resin 1 (AHEW 37.6) adduct Urea Catalyst Omicure ™ U410 Toluene bis-Dimethyl Urea (TBDMU, CVC Thermosets, Moorestown, NJ)

Example 1A

Synthesis of Epoxy-Amine Adduct from Tables 1 and 2

(7) Calculated amounts of cycloaliphatic amine and epoxy resin 1 (9:1 molar ratio of amine groups to epoxy groups) were weighed into a three necked round bottomed flask equipped with a magnetic stir bar, a thermometer and a nitrogen gas inlet. The flask was placed into a heating mantle. The mixture was stirred at 80° C. for 3 hours. The disappearance of epoxy functionality was confirmed using FTIR spectroscopy, peak at 915 cm.sup.−1

Example 1B

Formation of Thermosetting Resin Mixture from Table 2

(8) A hardener mixture was prepared first by weighing the amounts of amine adduct and dicyandiamide indicated in Table 1, above, into a speedmixer. The cup was then inserted into a dual asymmetric centrifugal FlackTek™ speedmixer (FlackTek Inc., Landrum, S.C.) and the contents were mixed for 2 minutes at 2000 rpm. Mixing was repeated in 2 minute cycles until the dicyandiamide dissolved completely and the solution became clear. Next, the thermosetting resin mixture of epoxy resins and the hardener mixture in the amounts indicated in Table 1, above, were weighed into a separate speed mixer cup and blended in the speedmixer by adding the hardener to the epoxy resin an mixing for 2 minute at 2000 rpm.

Example 2

Laboratory Infusion of Carbon Fiber Fabric

(9) Lab infusion experiments were carried out on an Alpha Technologies APA2000 rheometer (Alpha Technologies Inc., Bellingham, Wash.), which is a “moving die” parallel plate rheometer having an upper plate and a lower plate. Infusing comprised applying downward pressure via a pneumatically-actuated upper plate to a resin-carbon fiber fabric specimen within a die chamber equipped with an o-ring seal (epoxy-resistant EPDM-containing o-rings, size 2-129, compound EP692-45, Zatkoff Seals, Saginaw, Mich.) to prevent resin outflow while allowing air (gas) to escape the die chamber as the upper plate travels downward. In the rheometer, downward travel of the upper plate both compresses the specimen and infuses resin into the fabric.

(10) The following materials were used to make the specimen: 50 micron (2 mil)×5.12 cm in diameter plastic release film sheets; die-cut 39.35 mm diameter red silicone rubber backing disks, 0.79 mm thickness; masking tape, 50 micron (2 mil) thickness, die cut into 5.12 cm (2″) diameter disks, with a 2.24 cm (⅞″) diameter center punch removed; and die-cut 37.9 mm disks of 590 g/m.sup.2 non-crimp fabric (NCF) (+60°/−60°/0° layup; normal stitch; DowAksa AKSA CA A42 24K fiber; NCF product code=DMC4620305, from Sigmatex Carbon Textile Solutions, Inc., Orangeburg, S.C.).

(11) Infusion was performed with the rheometer set at a plate temperature of 60° C., wherein, a dry layup, including one NCF disk on a bottom silicone rubber backing disk and at bottom a release film sheet, was carefully centered on the bottom plate of the rheometer and heated for 2 min to bring the fabric to temperature. Then, 0.8 g of the thermosetting resin mixture indicated in Table 2, below, was applied onto the center of the exposed fabric and a sheet of the release film was laid on top of the resin and pressed gently to improve thermal contact for heating the resin. A silicone rubber backing disk was placed on top of the layup. After 30 seconds to preheat the resin mixture, the button was pushed to lower the upper plate and close the die and a 2 min countdown timer was started. Because fully closing the die takes about 15 sec at line pressure of 137.9 kPa (20 psig), the actual hold time at full pressure was less than 2 min. Last, the die was opened after the 2 min timer went off and the sample was promptly removed from the heated plate to cool.

(12) Test Methods: The following tests were performed:

(13) Differential Scanning calorimetry: Unless otherwise indicated, determined as defined above.

(14) Determination of Cured Tg: 10 grams of each of the indicated thermosetting resin mixtures prepared as described in Example 1B, above, were poured into aluminum weighing pans, and cured in an oven set at 150° C. for 3 minutes. The cured samples were then removed from the oven, cooled to room temperature after which ˜5 mg pieces were cut from each sample and tested using DSC to measure the cured Tg of the samples. The results are shown for Examples 3 and Comparative Example 3 in Table 3, below.

(15) Tg development of prepreg at room temperature: The prepreg Examples were stored at ambient temperature. Samples were cut at regular time intervals from the prepreg and tested using DSC to follow the Tg1 development of the prepregs over a period of 1 month. The results are shown in Table 4, below for the prepregs of Examples 4 and Comparative Example 4, which are, respectively, made from the thermosetting resin mixtures in Examples 3 and Comparative Example 3.

(16) The data in Table 2, below, shows that for thermosetting resin mixtures made as in Example 1B, above, an epoxy amine adduct will dissolve a greater amount of dicyandiamide in a smaller equivalent weight of amine.

(17) TABLE-US-00002 TABLE 2 Solubility of Dicyandiamide in Cycloaliphatic Amine vs. Epoxy Amine Adduct Comparative Example Example Example 3 3 Total Epoxy Resin* 88.5 87.82 MDACH 7.0 MDACH-Epoxy Resin 1 adduct 7.35 Dicyandiamide 1.85 2.20 Urea catalyst 2.65 2.63 Total 100.00 100.00 AHEW of MDACH/MDACH-LER 32 37.6 adduct % dicy dissolved in amine 21 24 Equivalents of MDACH/MDACH-LER 0.44 0.40 per 1 equivalent of epoxy *In Table 2, above, the Epoxy Resin comprises an 80/20 w/w mixture of Epoxy Resin 1 and Epoxy Resin 2.

(18) In the formulation of Example 3, at least 15% more dicyandiamide was dissolved in solution than in Comparative Example 3.

(19) TABLE-US-00003 TABLE 3 Cured Tg (Post cured for 3 min at 150° C.) Example Tg2 (° C.) Comparative Example 3 148 Example 3 153.6

(20) As shown in Table 3, above, a comparison of the thermosetting resin mixtures of the present invention versus the same resin mixture in Comparative Example 3 without the epoxy-amine adduct, the Example 3 thermosetting resin mixture has a higher cured of Tg>150° C.

(21) TABLE-US-00004 TABLE 4 Prepreg Tg Development at Ambient Temperature Tg1 (° C.) Example Day 17 Day 27 Comparative Example 4 30.6 32.2 Example 4 26.2 27.1

(22) The Initial Tg of the prepregs made from the formulation in Comparative Example 4 was ˜30-31° C. after two weeks, and stabilized at around 32-33° C. after nearly a month at ambient room temperature. The Tg1 of the prepreg from inventive formulation (Example 4) was about 5° C. lower under similar aging conditions.