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PAINT & PINTURA
Jan/Fev 2012
65
High Performance
Waterborne Epoxy System
and Formulations
Jim Elmore, Bedri Erdem, Dan Weinmann and Gilberto Saboia*
Momentive Specialty Chemicals
Abstract
Epoxy polymers are commonly used
in a variety of industrial protective
and maintenance coating appli-
cations because of their superior
adhesion, resistance to chemi-
cals and corrosion, and desirable
mechanical properties. Improved
performance and lower Volatile
Organic Compound (VOC) levels
in waterborne epoxy systems have
allowed the industry to replace
solvent-borne epoxies in many coa-
ting applications.
This article details an innovative
waterborne epoxy resin and curing
agent system developed by Momen-
tive Specialty Chemicals (former-
ly Hexion). Coating formulations
based on NewGen technology
offer low induction time, fast dry,
outstanding corrosion resistance,
improved ease of application, good
adhesion and good gloss at low
VOC levels. The excellent perfor-
mance of this two-component (2K)
waterborne epoxy system stems
from improved resin compatibility,
which improves coalescence, redu-
ces induction time, increases gloss
and speeds dry times and hardness
development.
Introduction
Epoxy resins based on the diglycidyl
ether of Bisphenol-A have afforded
good value for metal protection
since the 1950s. Ambient-cure
epoxy primers are commonly based
on solvent-borne epoxy resins with
high solvent content. Efforts to
reformulate these coatings to meet
environmental compliance requi-
rements (VOC regulations) over
the past thirty years have not met
expectations. Modifications have
compromised the coating’s metal
protection, diminished flexibility,
lengthened dry times, or reduced
the working pot life. Why?
Traditional 2K Epoxy /
Polyamidoamine Resin
Chemistry
The traditional binder system for
protective coatings is a “1-type”
Bisphenol-A epoxy resin and a tall
oil dimer acid-based polyamidoa-
mine resin. The polyamidoamine
curing agent is very hydrophobic
due to the tall oil dimer, affording
water resistance and good substra-
te wetting. The crosslinking betwe-
en the two long molecules imparts
flexibility and toughness.
The significant difference in solu-
bility parameter (
δ
) between the
epoxy resin and polyamidoamine
resin yields incompatibility. When
mixed, the epoxy resin resembles,
on a molecular level, a peanut
butter sandwich (Figure 1) whi-
le the planar and rigid Bisphenol
A epoxy aromatic ring backbone
forms the bread slices. The low T
g
,
amorphous polyamidoamine curing
agent eventually penetrates into
the layers of epoxy and after cross-
-linking, holds them together. The
molecular differences of the epoxy
resin and the polyamidoamine
resin lead to long induction times,
the possibility of blush formation
due to excess primary amines, and
high levels of solvent to offset the
limited compatibility of the poly -
mers.