Updated February 4, 2000
General Science Department
I. Test Purpose
This test is used to determine the length of time required to cure adhesive resin.
II. Summary of Test Method
A thin layer of adhesive resin, approximately 8 mm in diameter, is placed onto a clean microscope slide. Resin is cured with light, then wiped with an alcohol soaked tissue, & pressed & scratched with a dental explorer to determine if a firm layer has been established. By increasing or decreasing the cure time in steps (intervals), & checking for cure each time, the minimum cure time is determined.
III. Equipment and Materials
1. curing light
2. adhesive resin
3. dental explorer
4. Kimwipes
5. isopropyl alcohol
6. glass microscope slides
7. timer
8. 12X stereo microscope
9. air syringe
IV. Test method
Test Criteria:
For this test, adhesive is considered "cured" when it does not wipe away, & is firm enough that it withstands pressure without breaking up. This corresponds to the clinical situation of a clinician filling a preparation with restorative resin, using typical forces, & not pushing away or breaking up the adhesive layer previously placed.
This type of "wipe & press" test can be used for simple & fast comparisons of adhesive resins & curing lights, however, adhesive bond strength should also be tested using appropriate methods.
A. Sample Prep:
1. Apply adhesive to microscope slide following the manufacturer’s instructions. (Note: Etch & priming steps can usually be omitted.)
2. Position curing light tip over adhesive, within 3 mm.
3. Activate light & cure adhesive. Track cure time with timer.
B. Testing Degree of Cure:
1. Gently wipe adhesive with an alcohol soaked Kimwipe, then immediately wipe gently with a dry Kimwipe.
2. Examine adhesive under 12X microscope to determine how much remains after the wipes.
3. While viewing under 12X magnification, firmly press on adhesive surface with flat side of explorer to determine if adhesive will withstand pressure without breaking up or pushing away. Scratching & digging into adhesive with point of explorer can also help determine hardness.
4. Record sample number, adhesive brand name, curing light, cure time, hardness, & pass or fail on "cured". Hardness is described on the following scale:
a. wet or runny
b. starting to gel
c. gelled
d. soft or plastic
e. firm but pliable
f. hard but pliable
g. hard
h. very hard
Adhesive is considered "cured" when it does not wipe away, & is firm enough that it withstands pressure without breaking up. Typically, a hardness of level e or higher is sufficient. Some resins are softer than others, even when fully cured, so hardness alone is not always reliable.
C. Determining Minimum Cure Time:
Perform the test starting with a cure time of 1 second. Increase the cure time for each subsequent sample until the adhesive is cured. The sequence of samples formed should progress from uncured to cured, revealing the minimum cure time. Repeat the test for two or more additional sequences of samples. It is not always necessary to start with a 1 second exposure time. For example, if a previous sequence indicates that the cure time is 4 seconds, then subsequent sequences may consist of only two samples, using 3 and 4 second exposure times, showing that the resin is uncured after 3 seconds, but does cure after 4. No matter what exposure times are used in a sequence, the final minimum cure time determined must always be proven as such by a sample exposed for one time interval less, which results in uncured resin.
D. Establish Cure of Adhesive:
When working with a new adhesive, it may be helpful to follow the manufacturer’s recommended cure time with a halogen light first, so the characteristic hardness of cured adhesive can be examined, as well as the thickness of the oxygen inhibition layer prior to wiping.
E. Expression of Results:
Calculate the average cure time from the 3 or more tests and round up to the nearest time interval.
F. Precision:
Minimum cure time can be found to any desired accuracy. Typically, cure time is found to the nearest 1 second for cure times less than 10 seconds, & to the nearest 5 seconds for cure times greater than 10 seconds. Greater accuracy could be achieved by reducing the time interval to 0.5 or 0.1 seconds, however, as the interval is made smaller the influence of other variables increases.