Epoxy
Chemistry

HOME

PRODUCTS

PRODUCT
SELECTION CHART

HOW
TO / PUBLICATIONS

THE
105 SYSTEM

SPECIALTY
EPOXIES

FABRICS,
TOOLS & VACUUM BAGGINS

PHYSICAL
PROPERTIES

PRODUCT
LIST

HOW TO
USE

USE
GUIDES

VIDEOS

COURSES

PROJECTS

SAFETY

MSDS

GENERAL
SAFETY GUIDELINES

UNDERSTANDING
OVEREXPOSURE

HEALTH
EFFECTS FROM OVEREXPOSURE TO EPOXY

HOW
TO PREVENT OVEREXPOSURE TO EPOXY

OTHER
EPOXY RELATED HAZARDS

WORKSHOP
HAZARDS

ENVIRONMENTAL
CONCERNS

COMPANY

COMPANY
OVERVIEW

WHY
WEST SYSTEM

HISTORY

PRIVACY
POLICY

LINKS

WHERE
TO BUY

UK
STOCKISTS

UK
DISTRIBUTORS

INTERNATIONAL
DISTRIBUTORS

NEWS

CONTACT

Understanding cure time

Open time and cure time determine the build and repair
operations. Open time dictates the time available for mixing, application,
smoothing, shaping, assembly and clamping. Cure time dictates the time
before removing clamps, abrading or proceeding to the next step in the
project. Three factors determine the open time and cure time of an epoxy
mix – hardener cure speed, epoxy temperature and volume of mix.

a) Hardener speed

Each hardener has an ideal temperature cure range. At any
given temperature each resin/hardener combination will go through the same
cure stages but at different rates. Select the hardener that gives
adequate working time for the job in hand at the temperature and
conditions under which the work is to be completed. The Product Guide
describes hardener pot lives and cure times.

Pot life is a term used to compare the cure speeds of
different hardeners. It is the period of time a specific mass of mixed
resin and hardener remains a liquid at a particular temperature e.g. a
100g mass of an epoxy mix in a standard container at 25°C is a routine
quality control test procedure.

Because pot life is a measure of the speed of cure of a
specific mass (volume) of epoxy rather than a thin film, the pot life of a
resin/hardener mix is much shorter than its open time.

b) Epoxy temperature

The warmer the temperature the faster an epoxy mix will
cure (Figure 1). The temperature at which epoxy cures is determined by the
ambient temperature plus the exothermic heat generated by the
reaction.

Ambient temperature is the temperature of the air and/or
the material in contact with the epoxy. Epoxy cures faster when the
ambient temperature is warmer.

c) The volume of mixed epoxy

Mixing resin and hardener together creates an exothermic
(heat producing) reaction. Always mix small batches of epoxy because the
greater the quantity, the more heat generated, the shorter the pot life
and cure time. In a larger volume, more heat is retained, causing a faster
reaction and yet more heat e.g. a plastic mixing cup containing, say, a
200g mix can generate enough heat to melt the cup. However, if the same
quantity is spread into a thin layer, the exothermic heat is not produced
as quickly and the cure time of the epoxy is determined by the ambient
temperature.

Controlling cure time

In warm conditions use a slower hardener to increase the
open time. Mix smaller batches that can be used quickly or pour the epoxy
mix into a container with greater surface area e.g. a roller pan, thereby
spreading out the epoxy into a thin film and extending the open time.
After thorough mixing, the sooner the epoxy is transferred or applied, the
more open time is available for coating, lay-up or assembly.

In cool conditions use a faster hardener and employ a hot
air gun, a heat lamp or other dry heat source to warm the resin and
hardener before mixing and/or after the epoxy is applied. At room
temperature, additional heat is useful when a quicker cure is desired.
NOTE! Unvented kerosene or propane heaters can inhibit the cure of epoxy
and contaminate epoxy surfaces with unburned hydrocarbons.

CAUTION! Warming a
resin/hardener mix will lower its viscosity, allowing the epoxy to run or
sag more easily on vertical surfaces. In addition, heating epoxy applied
to a porous substrate (soft wood or low density core material) may cause
the substrate to “out-gas” and form bubbles in the epoxy coating. To avoid
out-gassing, wait until the epoxy coating has gelled before warming it.
Never heat mixed epoxy in a liquid state over 50°C.

Regardless of the steps taken to control the cure time,
thorough planning of the application and assembly will allow maximum use
of the open time and cure time of the epoxy mix.

Cure stages of epoxy

Mixing epoxy resin and hardener begins a chemical reaction
that transforms the combined liquid components into a solid. As it cures,
the epoxy passes from the liquid state , through a gel stage before it
reaches a solid state. (Figure 1)

1. Liquid – Open time

Open time (also working time) is the period, after mixing,
that the resin/hardener mix remains a liquid and is workable and suitable
for application. All assembly and clamping should take place during this
period to ensure a dependable bond is achieved.

2. Gel – Initial cure phase

The mix passes into an initial cure phase (also known as
the “Green Stage”) when it begins to gel. The epoxy is no longer workable
and will progress from a tacky gel consistency to the firmness of hard
rubber. An indent can be made with the thumb nail and it is too soft to
dry sand.

While the epoxy is tacky, a new application of epoxy will
chemically link with it, so the surface may be bonded or recoated without
sanding. This ability diminishes as the mix approaches the final cure
phase.

3. Solid – Final cure phase

The epoxy mix has cured to a solid state and can be dry
sanded and shaped. It is no longer possible to indent the surface with the
thumb nail. At this stage, the epoxy has reached 90% of its ultimate
strength, so clamps can be removed. The mix will continue to cure over the
next few days at room temperature.

A new application of epoxy will no longer chemically link
to it, so the surface must be thoroughly washed and sanded before
recoating to achieve a good mechanical, secondary bond. See Surface
Preparation.

Post curing

You can improve epoxy’s thermal performance and reduce the
potential for fabric “print-through” by applying modest heat to the epoxy
after it has cured to a solid state. Contact Wessex Resins for more
information about post curing epoxy.

NEED HELP!

Call our help line to talk with our technical staff,
weekdays 8:30 AM to 4:30 PM GMT

+44 (0) 8707 701030

WEST SYSTEM is a registered trademark of Gougeon Brothers,
Inc | Sitemap |
Terms
& Conditions |