Epoxy resin system containing polyethylene tetraamines for resin transfer molding processes

Abstract

A two-component curable epoxy resin system is disclosed. The resin system includes an epoxy resin component containing at least 80% by weight of a polyglycidyl ether of a polyphenol. The system also includes a hardener mixture containing mainly polyethylene tetraamines. The system includes one or more of i) alkali metal salts, ii) carboxylic acid-, carboxylic anhydride- or carboxylic acid ester-substituted phenol compounds, iii) an amino as a catalyst. The system has beneficial curing characteristics that make it useful for producing fiber-reinforced composites in a resin transfer molding process.

Claims

1. A process for forming a fiber-reinforced epoxy composite, comprising; a) mixing an epoxy resin component with an amine hardener by impingement mixing and transferring the resulting reactive mixture into a mold that contains reinforcing fibers, and b) curing the reactive mixture in the mold at temperature of 100 to 120 C. to form a fiber-reinforced composite in which the reinforcing fibers are embedded in a polymeric matrix formed by curing the reactive mixture, and c) demolding the fiber-reinforced composite, wherein; 1) the epoxy resin component contains one or more epoxy resins, wherein at least 80% by weight of the epoxy resin is one or more polyglycidyl ethers of a polyphenol that has an epoxy equivalent weight of up to about 250; 2) the amine hardener is a polyethylene tetraamine mixture containing at least 95% by weight polyethylene tetraamines, the mixture containing at least 40% by weight linear triethylene tetraamine, 3) in step a) the epoxy resin component and the amine hardener are mixed in proportions which, prior to reaction, provide 0.8 to 1.25 epoxy equivalents per equivalent of amine hydrogens; and 4) at least one catalyst selected from the group consisting of i) from 0.01 to 0.05 moles of an alkali metal salt per mole of the amine hardener, ii) from 0.025 to 0.25 moles of an carboxylic acid-, carboxylic anhydride- or carboxylic acid ester-substituted phenol compound per mole of the amine hardener, iii) from 0.025 to 0.25 moles of an amino acid per mole of the amine hardener is provided in the amine hardener, wherein the reactive mixture exhibits a gel time of at least 30 seconds and a demold time of no greater than 350 seconds measured at 100 C.

2. The process of claim 1, wherein the catalyst is lithium bromide.

3. The process of claim 1, wherein the catalyst is one or more of salicylic acid, methyl salicylate, ethyl salicylate, 4-hydroxybenzoic acid, methyl 4-hydroxybenzoate, ethyl 4-hydroxybenzoate, 4-hydroxyisophthalic acid, methyl 4-hydroxyisophthalate, and ethyl 4-hydroxyisophthalate.

4. The process of claim 3, wherein the catalyst is L-proline.

5. The process of claim 1, wherein the demold time is 300 seconds or less.

6. The process of claim 1, wherein the demold time is 240 seconds or less.

7. The process of claim 1, wherein the cured epoxy resin phase has a glass transition temperature of at least 110 C. at the time of demold.

8. The process of claim 1, which is a resin transfer molding process.

9. The process of claim 1, wherein the polyethylene ttramine mixture contains no more than 0.3% by weight aminoethylethanolamine.

10. The process of claim 1, wherein the reactive mixture contains an internal mold release agent.

11. A cured fiber-reinforced composite made by the process of claim 1.

Description

EXAMPLE 1 AND COMPARATIVE SAMPLE A

(1) The curing characteristics of curable epoxy resin system Example 1 and Comparative Sample A are evaluated as follows. Epoxy Resin A (a diglycidyl ether of bisphenol A having an epoxy equivalent weight of about 180 and a molecular weight of about 350) is mixed with a commercially available triethylene tetraamine amine hardener (D.E.H. 24 from The Dow Chemical Company) and the catalyst for 30 seconds. The triethylene tetraamine hardener contains about 99% by weight triethylene tetraamines of which about 68% by weight is linear triethylene tetraamine (H.sub.2NCH.sub.2CH.sub.2NHCH.sub.2CH.sub.2NHCH.sub.2CH.sub.2NH.sub.2).

(2) Ratios of starting materials are as indicated in the Table. A sample of the mixture is poured onto a preheated hot plate. The curing temperature is as indicated in Table 1.

(3) Time is measured from the point at which the mixture contacts the hot plate surface. A line is scored through the liquid disk periodically, using a pallet knife or similar blade. In this test, the gel time is the time after which the liquid material no longer flows into the scored line. Demold time in this test is the time after pouring at which the disk can be removed from the hot plate surface as a solid, using the pallet knife or similar blade. Results are as indicated in Table 1 below.

(4) Another portion of each of the epoxy resin systems is evaluated for glass transition temperature by dynamic scanning calorimetry as follows: the sample is heated from 25 to 250 C. at 20 C./minute, held at 250 C. for three minutes and cooled to 25 C. at 20 C./minute. The sample is maintained at 25 C. for three minutes and then heated again to 250 C. at 25 C./minute. Glass transition temperature is measured on the second heating segment, and taken as the temperature at the onset of the transition. Results are as indicated in Table 1.

(5) TABLE-US-00001 TABLE 1 Example or Comparative Sample Designation A* 1 Ingredient Epoxy Resin, eq. 1 1.1 TETA, eq. 0 1 DETA, eq. 1.1 0 LiBr, moles/mole TETA 0 0.02 Bisphenol A Mannich base, moles/mole DETA 0.1 0 Salicylic acid, mole/mole TETA 0 0 Test Results Temperature, C. 95 100 Gel time, s 45 90 Demold time, s 120 240 T.sub.g, C. 104 121 *Not an example of this invention. The epoxy resin is a diglycidyl ether of bisphenol A. TETA is a commercial triethylene tetraamine hardener sold as D.E.H 24 by The Dow Chemical Company.

(6) As can be seen from the data in Table 1, LiBr is an effective catalyst in this system. It provides a desirably long gel time followed by a rapid cure that allows demold (at the 100 C. curing temperature) within four minutes. The LiBr-catalyzed system produces a higher T.sub.g than the comparative sample.

EXAMPLES 2 TO 7 AND COMPARATIVE SAMPLES A AND B

(7) The curing characteristics of curable epoxy resin system Examples 2 to 7 and Comparative Sample B are evaluated in the same manner described in Example 1. The formulations of the epoxy resins systems are indicated in Table 2 below. Gel and demold times and glass transition temperature are as reported in Table 2. For convenience, the formulations and results for Comparative Sample A are repeated in Table 2.

(8) TABLE-US-00002 TABLE 2 Example or Comparative Sample Designation Ingredient A* B 1 2 3 4 5 6 Epoxy Resin, eq. 1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 TETA, eq. 0 1 1 1 1 1 1 1 DETA, eq. 1.1 0 0 0 0 0 0 0 Salicylic acid, mole/mole 0 0 0.05 0.1 0.2 0 0 0 TETA Methyl salicylate, 0 0 0 0 0 0.05 0.1 0.2 moles/mole TETA Bisphenol A Mannich base, 0.1 0 0 0 0 0 0 0 moles/mole DETA Test Results Temperature, C. 95 100 100 100 100 100 100 100 Gel time, s 45 105 105 90 75 90 60 60 Vitrification time, s 120 540 240 240 210 240 210 180 T.sub.g (onset), C. 104 101 N.D. 104 N.D. N.D. 110 N.D. N.D. = not determined. *Not an example of this invention.

(9) As can be seen from the data in Table 2, salicylic acid and methyl salicylate each are effective catalysts in this system. Each provides a desirably long gel time followed by a rapid cure that allows demold (at the 100 C. curing temperature) within four minutes. The systems catalyzed with these catalysts produces a higher T.sub.g than seen in the comparative samples.

EXAMPLES 8-10

(10) The curing characteristics of curable epoxy resin system Examples 8-10 are evaluated in the same manner described in Example 1. The formulations of the epoxy resins systems are indicated in Table 3 below. Gel and demold times and glass transition temperature are as reported in Table 3. For convenience, the formulations and results for Comparative Samples A and B are repeated in Table 3.

(11) TABLE-US-00003 TABLE 3 Example or Comparative Sample Designation A* B 1 2 3 Ingredient Epoxy Resin, eq. 1 1.1 1.1 1.1 1.1 TETA, eq. 0 1 1 1 1 DETA, eq. 1.1 0 0 0 0 L-proline, moles/mole TETA 0 0 0.05 0.1 0.2 Bisphenol A Mannich base, moles/ 0.1 0 0 0 0 mole DETA Test Results Temperature, C. 95 100 100 100 100 Gel time, s 45 105 90 75 75 Vitrification time, s 120 540 300 240 210 T.sub.g (onset), C. 104 101 N.D. 114 N.D. N.D. = not determined. *Not an example of this invention.

(12) As can be seen from the data in Table 3, L-proline is an effective catalyst in this system. It provides a desirably long gel time followed by a rapid cure that allows demold (at the 100 C. curing temperature) within four minutes. The proline-catalyzed system produces a higher T.sub.g than the comparative examples.