Case study: Moisturizer texture measurement

PRINCIPLE OF THE TEST

Evaluation of the consistencies of two types of moisturizers by back extrusion.

CONTEXT

Moisturizers are used to keep the skin smooth, soft, and looking radiant and healthy by retaining moisture or water in the outermost layer of the skin. Different moisturizers will be defined by their viscosity or their thickness. The formulation of moisturizers therefore largely depends on the required consistency of the final product, influencing the choice of material to be used.

A desirable factor in product development is to have a product that flows easily from a tube and breaks cleanly after being squeezed. With the use of front and rear extrusion tests, these consistencies can be assessed. The forward extrusion simulates the force required by the consumer to extrude the sample and the backward extrusion an indication of failure of product structure and flow properties. The rear extrusion attachment is useful for testing viscous products. Samples can also be tested in their containers directly from the production line.

METHOD

Equipment : CT3 with 4.5 kg load cell

Luminaire base table (TA-BT-KIT)

Double extrusion cell (TA-DEC)

TexturePro CT software

Settings :

Type of test: Compression

Pre-test speed: 1.0 mm / s

Test speed: 2.0 mm / s

Post-test speed: 2.0 mm / s

Target type: Distance

Target value: 25 mm

Trigger force: 10 grams

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

The chosen target distance should be such that the probe does not distort the sample more than 75 % depth of the sample, otherwise the base effect may affect the results.

PROCEDURE

Secure the extrusion disc to the load cell.

Install the rear extrusion cell on the base table; tighten the screws, but still allow some degree of mobility.

Align the extrusion container in the center under the extrusion disc (piston), then tighten the base table screws.

Remove the sample kept at a specific temperature from storage.

Fill the extrusion container with the sample up to about 75 % of its capacity, minimizing air bubbles.

Position the probe at a specified starting distance, for example 10 mm above the top of the vessel or the surface of the sample. This will ensure that the probe returns to the same position after each test, allowing comparisons of the adhesion strength which is an indication of the cohesion of the sample.

RESULTS

Comparison of the consistencies of two types of moisturizers by back extrusion.

The graph in Figure I shows the consistencies of two types of moisturizers stored and tested at 21 ° C in a 40mm diameter rear extrusion vessel.

Data Set # 1: Sample A (Premium Moisturizer)

Data Set # 2: Sample B (Value Moisturizer)

OBSERVATIONS

When a surface trigger of 10g is reached, the plunger pushes on the sample to a depth of 25mm. During this time, the sample is deformed and compressed to settle more tightly in the decreasing available space (under the descending piston) and the force increases steadily. As the sample becomes more compact with limited air pockets, the force increases sharply and extrusion begins. As the force increases to a maximum point, a plateau is observed indicating the force required to continue extrusion. This is the final charge.

The maximum force on the graph is a measure of firmness; the higher the value, the firmer the sample. The area below the positive part of the graph indicates the consistency of the sample (work performed up to hardness1); the higher the value, the thicker the sample and the higher the consistency.

When the probe returns to its starting position, the initial lift of the sample weight onto the top surface of the disc produces the negative portion of the graph resulting from the back extrusion. This gives an indication of the tack / cohesion and resistance of the sample to separate (flow) from the disc.

The maximum negative force on the graph indicates the adhesive strength of the sample; the more negative the value, the more “sticky” the sample. The area below the negative part of the graph is known as adhesion (the energy required to break the contact of the probe with the sample) and can give an indication of the cohesive forces of molecules in the sample. The higher the value, the more energy it takes to break the contact of the probe with the sample when the probe withdraws from the sample.

From Figure I, samples A and B have similar maximum peak (firmness) values, but sample A has a higher consistency, visible through the larger area under the positive curve than sample B. Sample B shows a change in consistency as shown by the initial stability increase in force during the first few seconds followed by a more rapid increase in force indicating the possibility of sedimentation in the sample and the formation of two layers. Sample A has a higher adhesive component than Sample B with a higher negative peak value.

The test can be used as a quality control tool to ensure consistency in production as well as in R&D and product development by monitoring the effect of formulation or process control changes. The tests obtained from 3 moisturizers of each formulation give the values of average hardness and work carried out below: