Application
Cosmetics: Lipstick

 Introduction

Lipsticks are a complex mixture of soft and hard fatty compounds (beeswax,ozokerite, lanolins…), polymers, pigments, perfumes and
conservation agents. Two of the most common problems oflipsticks are the following:
• Exudation of oil droplets on the surface;
• Crystal formation of waxy compounds on the surface;
Both problems result in a deteriorated aspect of the product (see beside), which leads to complaints from the customer.
In order to validate a lipstick formulation, long and tedious temperature cycle tests are made. For instance, lipsticks
are heated to 50°C for 12 hours andthen cooled down to 4 °C  for another 12 hours. These cycles are repeated over 30 days.

This application note wil show how the RHEOLASER Crystal can improve the formulation of lipsticks by an accelerated ageing test. 

 Objective

Measure the melting properties of margarines

Device

Rheolaser Crystal

Reminder on the technique 
RHEOLASER Crystal uses the DWS principle. 

Light is scattered by the particles, creating an interference pattern (Speckle Image). The variation of this image is related to the motion of the particles. By a mathematical analysis of this variation, decorrelation functions can be calculated and then processed, to obtain a characteristic time τ as a function of time or temperature.
Values of 1/τ or Micro-Dynamics (Hz), are then plotted against time or temperature, resulting in characteristic peaks when the product shows a microstructural evolution, such as a phase transition or any other physical event. The signal can then be integrated for an easier visualisation, obtaining the so-called Micro-Dynamics Evolution (%).

 Experimental Results

1 Effect of aging on the Micro-Dynamics signature

Two lipsticks with the same composition were compared with the Rheolaser Crystal:

Lipstick out of the production line (L-1)

Lipstick tested 30 days after ra temperature aging test (L-2)

A heating ramp was applied in the instrument to lipstick L-1, which was never heated since production. The figure on the right shows the Micro-Dynamics of L-1 during the heating ramp ( 10 to 90 °C, 1 °C/min). The sample did not change between 10 and 50 °C, indicating that there is no fusion or melting, or any other thermally induced transition of fatty compunds and polymers.

The figure on the left shows the same product ,which was thermally aged for 30 days by alternating temperature from 4 to 50 °C (Sample L-2). During the heating ramp on the instrument , a new peak is observed between 30 and 40 °C. Even if the lipstick is not melted completely by this temperature ramp , the energy input allows migration and segregation of the commonly crystallized waxes(eutectic crystal). The new peak is attributed to some of the fatty compounds meltingh at a lower temperature. 

2 Effect of Accelerated aging (short temperature cycles)

The figure below shows the superposition of the Micro-Dynamics during a ramp of 10 to 90 °C on the same lipstick, but with three diffrent thermal treatements prior to the experiment.

L-1 (no thermal treatment ; black)

L-2 (thermal treatment for 1 month:red)

L-1 (artificially aged with 5 short thermal cycels
( 1 h @ 10°C, 1H @40 °C: blue)

 During the heating ramp of L-1 (artifically aged ) a peak appeared between 50 and 60 °C, which is in an intermediat temeprature range between the peak observed for the long term tested lipstick and the lipstick out of the production line.

The lipstick has therefore changed structure after the 5 short cycles of heating and cooling. The shift of the peak towards the lower temperature indicates a lower temperature resistance and the lipsytick may show crystallixzation and/or exudation in the future. Combining these short temperature cycles with the RHEOLASER  Crystal measurements allows us to anticipate the future stability of the lipstick.

A 3rd lipstrick L-3 , which has never shown any crystals or exudation on the surface after long-term experiments , was also tested. The figure on the left shows the result of the heating ramp for the origional lipstricl L-3 ( black) out of the production line(no aditional thermal treatment) and the same lipstrick artificially aged  for 5 cycles (1 h @ 10°C, 1H @40 °C: blue). In this case the thermally aged sample shows the same profile as the untreated sample.

This allows us tro predict that this lipstick L-3 will not show any change in the future , such as exudation or crystal formatiom. 

Rheolaser Crystal offers:

PREDICTIVE – Anticapte long term phemomena

ACCURATE - nano-scale sensitivity, large sample, direct sampling with no denaturation

NO STRESS – analize fragile samples without changing the crystalline structure

Summary

 RHEOLASER  Crystal can be used as a straightforward tool in accelerated thermal stress studues for the evalutation of the long-term stability og lipsticks. The shift of the characteristic peaks to a lower temperature indicates possible exudation ro crystallization issues. This techniqu can be used for the optimisation of new formulations or in quality control.  

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