Lipstick

TEST PRINCIPLE
The objective was to compare the bending force of two brands of lipsticks; premium lipstick and budget lipstick.  

BACKGROUND
For users of lipsticks, there is a general preference for a product that does not bend, crumble, crack, or break during application.  As such, base ingredients become of paramount importance in formulating a product with resilience and physical strength capable of withstanding firm lipstick application by the user.

The cantilever test is an imitative test using a hemispherical edged probe to simulate the bending action caused during application.

METHOD

Note: It is recommended that the pre-test speed be the same as or less than the test speed for accurate trigger detection; for example, 1 mm/s test speed will require ≤ 1 mm/s pre-test speed.

SAMPLE PREPARATION
Condition the sample for temperature before testing. In this test samples were conditioned at 21°C overnight. 

PROCEDURE

1. Secure the fixture base table to the base of the machine and fit the base plate of the cantilever onto the fixture base table. 
2. Clamp the sample within the orifice of the supporting fixture.
3. Wind out the lipstick to its maximum length
4. Position the upper bending fixture such that it strikes the sample at approximately 3-4mm away from the tip. This will enable the sample to bend in response to the force applied by the lip shaped hemispherical probe emulating lipstick application to the lips. 

RESULTS
A typical TexturePro CT plot of the bending force on two different lipsticks:

Figure I

The graph in Figure I shows the bending force of two different 4 g lipsticks tested at 21°C and 3 mm from the tip of the lipstick. 

Data Set #1: Sample A (Premium Lipstick)
Data Set #2: Sample B (Budget Lipstick)

Figure II 

The Figure II graph shows the load verses distance for the bending force of two different 4 g lipsticks.

Data Set #1: Sample A (Premium Lipstick)
Data Set #2: Sample B (Budget Lipstick) 

OBSERVATIONS
When a trigger force of 20 g is achieved, the probe begins to move down to a deformation distance of 7 mm.  During this time the sample is bent until it breaks away (in some cases).

Sample hardness can be obtained from the maximum force value on the graph and sample stiffness (apparent modulus) from the gradient of the slope during the bending action.  The distance at which the break occurs (deformation at peak load) gives an indication of the brittleness of the sample.  Typically, a brittle sample would posses a short bending distance.  Energy required to bend the lipstick can be quantified by selecting (work done hardness1 in the software), this also gives a good indication of the consistency of the lipstick.  From the graph, Sample A (premium lipstick) is twice as hard as Sample B (budget lipstick).  Sample B is also more brittle than Sample A breaking off within two seconds.

Test results from two lipstick samples give the following values: