which evaporates, leaving a thin film of the pure compound. In actual use in resins, the polyamines and derivatives are in very dilute concentration, and irritability suffers a logarithmic decrease with dilution; a 10% concentrate is only a hundredth as irritating as one of full strength.
I
A. K. INGBERMAN R. K. WALTON
Low-Toxicity Aliphatic Amines a s Cross-linking Agents
for Polyepoxy Resins
W
O R m R s using epoxy tooling ‘compounds can soon do their jobs with no fear of severe skin irritation or allergic reaction to polyamine hardeners. The solution to the problem which has been plaguing tool and die producers and electrical equipment manufacturers is simply based on the introduction of a hydroxyalkyl group into the structure of the amine cross-linking agent. Basically the problem has been limited to processes requiring production workers to come in direct contact with epoxies curing a t room temperature. Most epoxy castings fall in this temperature group-only about 20% use high temperature cures. Research has shown that it is generally necessary to use aliphatic polyamines possessing a t least three hydrogens on nitrogen as the crosslinking agent to obtain a thermosetting, three-dimensional polymer at room temperature conditions. A typical compound of this type is diethylenetriamine (DETA), widely used industrially for this purpose. The problem has become aggravated as more and more nonchemists, who do not basically understand the problems of toxicity and sensitivity, use these epoxies. In the course of their jobs, workers may have to climb all over a die several feet long, coming into considerable direct contact with the epoxies. In some cases, gloves or a hood cannot be used, and workers must be near enough to do the work from a practical standpoint. The polyamines such as diethylenetriamine do not threaten systemic a small amount of toxicity-drinking
the liquid or breathing some of the vapors is not injurious. But where the human body does not have a chance to oxidize the chemical-a splash in the eye, penetration of vapor or liquid through the skin-the danger is very significant. They are among the most potent primary skin irritants, causing edema and, in severe cases, necrosis. This skin irritation is frequently complicated by a high incidence of allergic skin sensitization in a manner analogous to hay fever, or in some cases to poison ivy. For most people, initial contact seems to cause no ill effect, but after a period of contact, the individual level of irritation is reached and reaction is noticed. Once the process begins, the “threshold level” drops, and much smaller amounts cause reaction. Tests show that the adverse sensitivity is due neither to alkalinity nor to volatility within this group of polyamines. However, introduction of a hydroxyalkyl group had marked effect on these-, properties. Diethylenetriamine sensitizes about three quarters of the people that regularly are in contact with it. Pharmacological tests with the compounds a t Union Carbide’s medical laboratories at Mellon Institute show that hydroxyalkylated derivatives of ethylenediamine sensitized the skin of about a third of the guinea pigs tested, while the hydroxyalkyl derivatives of diethylenetriamine were free of sensitizing action on the animals. Diethylenetriamine itself sensitized all the animals tested. Skin tests are generally made with a 10% solution of the material in ethyl alcohol,
The desired properties of a crosslinking agent for polyepoxy resins include minimum irritation potential, rapid cure a t room temperature ambient conditions, long working life, ready miscibility with the resin, and optimum physical properties in the cured state. With the introduction of the hydroxyalkyl polyamine hardener as reported by Bakelite Co., the first requirement seems to be satisfied. I t remained, however, to show that the hydroxyalkylation of these polyamines had no adverse effect on their use as crosslinking agents. To this end, several of the newly designed hardeners were compared with conventional hardeners in terms of formulating, curing, and endproduct characteristics. From the standpoint of physical properties of the end product, the studies conducted on the minimum irritation potential hardener were evaluated on the basis of hardness, impact, flexural, tensile, and compressive values. I t was, of course, possible that introduction of the hydroxyalkyl group would have a degrading effect on these properties. In practically every instance, however, tests verified the fact that the new hardeners were equal to or better than the conventional cross-linking agents. Cure time of the resin hardener mixture was found to be reduced slightly by the presence of terminal methyl groups, which caused increased steric hindrance. O n the other hand, the presence of these terminal methyl groups improved miscibility of the resin with the cross-linking agent. Working life properties are similar to those exhibited by the conventional hardeners. On the basis of these and other test results, Bakelite is marketing the low toxicity hardener as a combination of 30 parts of bisphenol A and 100 parts of N-hydroxyethyldiethylenetriamine.This type of product, long needed in the industry, should help remove some of the obstacles holding back a further development of applications in the epoxy field.
C. F. PlTT M. N. PAUL Bakelite Co., Bound Brook,
N. J.
Division of Paint, Plastics, and Printing Ink Chemistry, Symposium on Epoxy Resins, 130th Meeting, ACS, Atlantic City, N. J. VOL. 49, NO. 7
JULY 1957
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