Robert Kultzow TRFA 2005 November 15, 2005 Epoxy Systems For Below Zero Degrees Celsius

Features of Epoxy Resins High mechanical strength Outstanding dielectric characteristics Excellent adhesive properties Great Chemical resistance Phenomenal thermal endurance

Performance at Lower Operating Temperatures Speed and effectiveness of cure Fracture toughness Thermal Expansion characteristics

Uses of Epoxies at Lower Temperatures and Cryogenic Conditions Nuclear physics apparatus Super conducting devices comprised of magnets and transformers Magnetic imaging devices

Pathways to Development of a Cryogenic System

Epoxy Systems That Exhibit Excellent Cryogenic Performance System ASystem A 100pbw - Modified Bis-A Epoxy 57pbw - Hardener A 10 pbw - Cycloaliphatic Diamine System BSystem B 100pbw – Modified Bis-A Epoxy 15pbw – Hardener A 37pbw – POPDA (High Molecular Weight) 20pbw – POPDA (Low Molecular Weight) 10pbw – Cycloaliphatic Diamine

Properties of A and B Cryogenic Systems PropertySystem ASystem B Viscosity, cps, 25°C6301,000 Gel time, min., 25°C9901,200 Barcol Hardness Thermal shock, cycles >25 Impact strength, Nm/mm 80°K Flexural strength, 4.2°K 12,325 40,555 29,435 4,640 23,200 _ Flexural modulus, 4.2°K 391,000 1,044,000 1,102, ,500 1,059,000 -

Thermal Shock Specimen Epoxy Steel Bolt

Gel Time vs. Cure Speed Gel Time is defined as the required time for a system to make an exothermic state change from liquid to solid. Cure speed is the time it takes for a system to actually cross link with itself in order to form a lattice structure.

Low Temperature Curing Phenalkamines - excellent for low temperature curing POPDA – gives excellent properties Accelerators such as benzyl alcohol, salicylic acid, and dimethylaminopropyl- amine PropertyAmine A Phenalk- amine Gel time, min., 25°C 6650 Pencil Hardness 3H Cure through time (5°C) >24 hours16 hours Direct Impact Test (in/lb) 1412

Cracking of Epoxies in Structural Applications Epoxies crack in many electrical apparatus due to sudden changes in temperature. Cracks usually start in areas of high stress High stress areas include places where a metal or ceramic insert is placed.

Fracture Toughness This is measured by calculating K Ic and G Ic of a material. The above figure illustrates different modes of fracture testing The below figure illustrates a double torsion method used on filled materials [ K1c] 2 = E* G 1c * (1-ν)

Toughening Concepts Incorporating crack- arresting micro-phases such as fillers, short fibers, micro-voids, glass beads, thermoplastics, and rubbers Matrix flexibilization MaterialG 1c [J/m 2 ] Pure metals 1,000,000 Steel 100,000 Titanium alloys 53,000 Aluminum alloys 30,000 Polypropylene 8000 Polyethersulfone 2500 Rubber toughened-epoxy 2000 Polycarbonate 800 Bis-Aepoxy / DDS 250 Marble 20 Window glass7

Core-shell Toughening Incorporates a fine dispersion of soft particles as a second phase within the epoxy matrix Such particles, with sizes less than 1 micron have a core structure that absorbs energy and a shell that provides for good adhesion to the epoxy matrix.

Core-Shell Morphology

Testing Crack Resistance

Thermal Cycle Soak Test

Results of Soak Testing

Conclusions Epoxies noted for: Excellent mechanical strength Outstanding dielectric properties Excellent chemical resistance Increased usage in medium and high voltage applications where subject to hostile environments

Conclusions Different approaches are available to formulators to improve toughness critical in low temperature applications Matrix flexibilization Multiphase toughening