Cross-Linked Polymers - American Chemical Society

Chapter 17

Recycling of Cured Epoxy Resins

Downloaded by UNIV OF MISSOURI COLUMBIA on June 15, 2013 | http://pubs.acs.org Publication Date: April 18, 1988 | doi: 10.1021/bk-1988-0367.ch017

Conchita V. Tran-Bruni and Rudolph D. Deanin Plastics Engineering Department, University of Lowell, Lowell, MA 01854

Epoxy resin was cured with polyamine, polyamide, or anhydride; ground to pass a 0.5-mm screen; and 20% was recycled into the next batch of virgin resin, either directly or after pre-soaking in the liquid ingredients. When these "filled" resins were cured, hardness was generally higher than for virgin resins. For polyamine-cured resin, recycle of pre-soaked epoxy also increased both impact strength and heat deflection temperature. For polyamide-cured resin, recycle also increased volume resistivity. Pre-soaked recycle dramatically increased adhesion to aluminum, especially in the anhydride system. Urbanization and industrialization of the world are producing increasing amounts of s o l i d waste, and disposal of this s o l i d waste i s becoming a growing problem; the rapid growth i n usefulness and production of plastics i n our society i s making plastics a growing portion of this t o t a l s o l i d waste (1). Thermoplastic waste i n manufacturing can generally be recycled by blending i t homogeneously with virgin thermoplastic material, with l i t t l e or no s a c r i f i c e i n properties and usefulness (2-4); and even collection, purification, and recycling of post-consumer waste i s becoming practical i n selected f i e l d s such as polyethylene terephthalate and high-density polyethylene bottles and polypropylene battery cases (£). Thermosetting plastics offer many advantages i n end-use properties for high-performance applications (6«?). They suffer from greater d i f f i c u l t y i n processing, and from the low u t i l i t y of manufacturing scrap, which therefore becomes s o l i d waste, and constitutes a double economic burden. The scrap can of 0097-6156/88/0367-0237$06.00/0 © 1988 American Chemical Society In Cross-Linked Polymers; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


238

CROSS-LINKED

P O L Y M E R S

course be ground and blended with v i r g i n resin; but since i t i s insoluble and infusible, this creates more problems than i t solves. I t invariably increases viscosity and makes processing more d i f f i c u l t . I t can be used as a s o l i d f i l l e r ; because of i t s chemical similarity to the virgin resin, i t might be hoped that i t would give a stronger i n t e r f a c i a l bond and thus reinforce properties; but i n most cases, the thermoset scrap i s so chemically resistant ( i . e. unreactive) that the v i r g i n resin cannot bond to i t successfully. On the contrary, since grinding of the r i g i d glassy thermoset scrap produces particles with sharp edges and sharp corners, these act as stress concentrators i n the cured v i r g i n resin and thus only weaken and embrittle i t . Therefore recycle of thermoset scrap has rarely been successful. About 20% of vulcanized rubber scrap i s recycled by thermal/chemical cleavage of cross-links to make i t melt processable again (8 ). Experimenters have reported that they can grind and recycle moderate proportions of thermoset polyesters, polyurethane rubber, and phenolic resins ( ? - l l ) into virgin material, and sometimes even observe some reinforcement of properties. The present study was undertaken to explore the p o s s i b i l i t y of grinding and recycling cured epoxy resins back into virgin epoxy resin formulations, particularly by presoaking the ground scrap i n the v i r g i n liquids i n the hope of penetrating the thermoset particles and thus bonding them more firmly into the v i r g i n matrix during the ultimate cure reaction. Experimental Shell Epon 828 epoxy resin was cured with 13 PHR (parts per hundred of epoxy) of triethylene tetramine as a t y p i c a l polyamine, 65 PHR of Shell Epon V-15 polyamide as a t y p i c a l polyamide, or 130 PHR of dodecenyl succinic anhydride as a typical anhydride. Liquid formulations were cast i n aluminum molds and oven-cured as follows: Polyamine: 1 Hr./100°C Polyamide: 1 Hr./100°C Anhydride: 1% Benzyl Dimethyl Amine Accelerator, 1 Hr./120°C +2.5 Hr./150°C. This produced test specimens directly. For recycle, cured specimens were cut up with a band saw, mixed with dry i c e , and ground i n a Fitzpatrick Homoloid WD-36-3 hammer m i l l to pass a 500-micron screen; 70-00% of the ground recycle was between 200-500 microns, the remainder f i n e r . Twenty percent of recycle was mixed with 80% of the corresponding v i r g i n formulation, and cured the same way as before. When the recycled resin

In Cross-Linked Polymers; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


17.

T R A N - B R U N I & D E A N I N

Recycling

of Cured Epoxy

Resins

239

was simply added to the l i q u i d system just before casting and curing, i t was referred to as "Dry" f i l l e r . Alternatively, the recycled resins were pre-soaked i n v i r g i n l i q u i d s , 1 Hr./90°C + 4 Days/Room Temperature, i n the hope of "activating" them, before mixing, casting, and curing the formulations. Polyamine- and polyamidecured recycles were soaked i n v i r g i n epoxy resin; anhydride-cured recycle was soaked i n v i r g i n anhydride curing agent. These systems, containing "Soaked" f i l l e r s , were cured the same way as before. Rockwell L hardness was measured according to ASTM D-785, f l e x u r a l modulus and strength according to D-790, dart impact strength according to D-3763 heat deflection temperature according to D-1637 at 264 PSI, and volume r e s i s t i v i t y according to D-257. Results are summarized i n Tables I-III. f

Discussion Recycle of a thermoset powder would be expected to resemble the use of particulate f i l l e r s . Thus we might expect i t to increase hardness, modulus, and heat deflection temperature, and decrease strength and impact resistance. On the other hand, i f similar polarity and/or chemical r e a c t i v i t y produced i n t e r f a c i a l bonding, we might hope that the recycle could act as a reinforcing f i l l e r , and help to retain or even improve strength and impact resistance; and we might further hope that presoaking the recycle i n the v i r g i n l i q u i d system would improve i n t e r f a c i a l bonding even more. The experimental results indicated that the recycle did generally increase hardness and decrease strength as expected. Effects on modulus ranged from negligible to negative, suggesting that the recycle either inhibited cure or simply introduced weak interfaces. Other effects were more selective. While recycle usually lowered impact strength and heat deflection temperature, pre-soaking polyamine recycle surprisingly improved both of these properties. While recycle usually lowered volume r e s i s t i v i t y , polyamide recycle improved i t . F i n a l l y , adhesion of epoxy formulations to the aluminum mold, i n spite of wax and silicone mold release agents, was dramatically increased by the use of presoaked recycle, especially i n the anhydride system, suggesting unexpected usefulness i n epoxy adhesive formulations. Such a variety of effects suggests that (1) Further analytical study would be required to understand them theoretically, but (2) Broader p r a c t i c a l study would pinpoint formulations which would permit the recycler to optimize c r i t i c a l end-use properties. This would be useful primarily i n epoxy moldings and castings, and i n adhesive formulations.



Soaked 113 +5 236 +13 5.76 +0.50 0.834~ 46 +1 0.87 ~

Soaked 128 +1 325 +9 6.67 +0.88 1.67 ~ 108 +1 0.024~

Dry Soaked 122 +1 126 +2 368 +13 244 6.84 +0.66 7.31 0.834""

Cross-Linked Polymers - American Chemical Society

Recommend Documents