Titanium Dioxide

Function of Titanium Dioxide in Rubber Shoe Manufacturing (Batch Production)

In rubber shoe manufacturing, TiO2 is primarily used for its exceptional properties as a pigment and a functional filler:

1. Coloring and Opacity: TiO2 is the most effective white pigment available. In rubber shoes, especially those with white or light-colored soles or uppers, it provides:
   • High Whiteness: Achieves a bright, clean white color.
   • Excellent Opacity: Effectively covers the natural color of the rubber and other ingredients, ensuring a consistent and uniform appearance.
   • Color Stability: Helps maintain the desired color over time, preventing yellowing or fading due to exposure to sunlight (UV radiation).
2. Reinforcement and Mechanical Properties: TiO2 acts as a reinforcing filler, improving the mechanical properties of the rubber compound. This contributes to:
   • Enhanced Tensile Strength: The ability of the rubber to withstand stretching forces without breaking.
   • Improved Tear Resistance: Resistance to the propagation of cuts or nicks, making the shoes more durable.
   • Increased Abrasion Resistance: Resistance to wear and tear from friction, prolonging the lifespan of the shoe soles.
   • Better Hardness: Contributes to the desired stiffness and resilience of the rubber.
   • Elongation Rate: While primarily a pigment, it can also influence the material’s ability to stretch before failure.
3. UV Resistance and Anti-Aging: TiO2 helps protect the rubber from degradation caused by ultraviolet (UV) radiation from sunlight. This is particularly important for shoes used outdoors, as it helps to:
   • Prevent Cracking: Reduces the likelihood of the rubber becoming brittle and cracking over time.
   • Maintain Flexibility: Helps retain the rubber’s elasticity and resilience.
   • Extend Product Lifespan: Overall, contributes to the longevity and performance of the rubber shoe.
4. Cost-Effectiveness: As a filler, TiO2 can optimize formulation costs while maintaining or even enhancing desired properties.

Both anatase and rutile forms of TiO2 are used in rubber, with rutile often preferred for its higher lightening power and durability, especially when enhanced UV and ozone resistance are needed (e.g., in automotive tires). For rubber shoes, the choice depends on the specific requirements for whiteness, opacity, and performance.

Laboratory Testing Parameters Before Batch Production

Before initiating batch production of rubber shoes, extensive laboratory testing of the rubber compound (which includes TiO2) is crucial to ensure the final product meets specified quality and performance standards. These tests typically fall into several categories:

A. Uncured Rubber Compound Testing (Processability):

1. Mooney Viscosity:
   • Purpose: Measures the flow properties (viscosity) of the uncured rubber compound. This is critical for assessing its processability during mixing, extrusion, and molding.
   • Parameter: Mooney viscosity value (ML 1+4 at 100°C is common).
   • Importance: Too high viscosity can make processing difficult and energy-intensive, increasing scorching risk. Too low viscosity can lead to stickiness and poor shape retention.
2. Cure Characteristics (Rheology):
   • Purpose: Evaluates the curing (vulcanization) behavior of the rubber compound using a rheometer (e.g., Oscillating Disc Rheometer – ODR, or Moving Die Rheometer – MDR).
   • Parameters:
     • Scorch Time (tS1 or tS2): The time until the onset of vulcanization, indicating how long the compound can be processed safely before curing begins.
     • Optimum Cure Time (t90): The time required to achieve 90% of the maximum torque, indicating the ideal curing time for optimal properties.
     • Maximum Torque (MH) and Minimum Torque (ML): Indicate the stiffness of the cured and uncured rubber, respectively.
     • Cure Rate: How quickly the rubber cures.
   • Importance: Ensures consistent curing, preventing under-curing (poor properties) or over-curing (brittle product).
3. Specific Gravity/Density:
   • Purpose: Measures the density of the uncured compound relative to water.
   • Importance: Aids in material characterization, composition analysis, and quality control, ensuring consistent formulation and yield.

B. Cured Rubber Testing (Performance & Durability):

After curing test specimens (often 2mm thick sheets vulcanized in a mold), the following physical and mechanical properties are typically tested:

1. Tensile Properties (ASTM D412):
   • Purpose: Measures the material’s response to stretching forces.
   • Parameters:
     • Tensile Strength (TS): Maximum stress the material can withstand before breaking.
     • Elongation at Break (Eb): Percentage increase in length before rupture, indicating flexibility and resilience.
     • Modulus (e.g., M100, M300): Stress required to achieve a certain percentage of elongation, indicating stiffness.
   • Importance: Crucial for shoe soles that need to withstand repeated bending and flexing without tearing.
2. Hardness (ASTM D2240 – Shore A/D):
   • Purpose: Measures the material’s resistance to indentation.
   • Parameters: Shore A or Shore D hardness value (e.g., 70 Shore A).
   • Importance: Determines the feel and performance of the sole (e.g., cushioning, grip, wear resistance).
3. Tear Strength (ASTM D624):
   • Purpose: Measures the material’s resistance to the propagation of a tear from a nick or cut.
   • Importance: Essential for shoe soles that may encounter sharp objects or experience concentrated stress points.
4. Abrasion Resistance (ISO 4649, DIN 53516):
   • Purpose: Assesses the material’s ability to withstand wear caused by friction.
   • Parameters: Volume loss in mm³.
   • Importance: Directly relates to the lifespan of the shoe sole, especially in high-wear areas.
5. Compression Set (ASTM D395):
   • Purpose: Measures the material’s ability to return to its original thickness after prolonged compression.
   • Parameters: Percentage of permanent deformation after compression.
   • Importance: Ensures that shoe components, like midsoles, maintain their cushioning properties over time and don’t permanently deform under pressure.
6. Aging Tests (e.g., Heat Aging, Ozone Resistance, UV Exposure – ASTM D573, ASTM D1148):
   • Purpose: Simulates the effects of environmental factors over time to predict the long-term durability of the rubber.
   • Parameters: Changes in tensile strength, elongation, hardness, and visual appearance after exposure to elevated temperatures, ozone, or UV light.
   • Importance: Critical for shoe soles that will be exposed to varying environmental conditions, ensuring they don’t degrade prematurely. TiO2’s role in UV resistance is directly assessed here.
7. Flex Cracking Resistance:
   • Purpose: Evaluates the material’s ability to withstand repeated bending without cracking.
   • Importance: Particularly relevant for shoe soles that undergo continuous flexing during walking.
8. Chemical Resistance:
   • Purpose: Determines the material’s compatibility with various chemicals it might encounter.
   • Importance: Relevant for specialized footwear or in environments where chemical exposure is a concern.

By conducting these comprehensive tests, manufacturers can ensure that the rubber compound, including the specific grade and amount of titanium dioxide used, is optimized for the desired aesthetic and performance characteristics of the rubber shoe before committing to large-scale batch production. This helps to minimize waste, ensure product quality, and meet customer expectations. Sources