Rubber Compounding & Mixing:

A Breakdown of How to Effectively Engineer

& Mix Rubber Formulations

By Cody David, West American Rubber Company

Rubber Compounding
Rubber compounding is the starting point; it defines the performance characteristics of rubber goods used in a wide array of
applications and industries. For rubber products to meet the requirements of an application, their essential physical characteristics
must be studiously engineered.

This intricate science involves the careful selection and blending of various ingredients to craft formulations that will meet the
industrial needs of the application. To ensure optimal performance, longevity and reliability of a rubber formulation must be
balanced effectively to create the correct set of properties, process efficiently, and mitigate costs.

1. Effective Rubber Formulations

Achieving the right balance of ingredients in rubber compounding goes beyond mere measurements. Truly, this is an art.

Rubber chemists leverage their expertise to meticulously select and blend components, refining recipes to optimize rubber
formulations for superior mechanical properties, and resilience against stress, strain, and environmental factors. The vast
variation of requirements that can be engineered in the final product showcases the adaptability and flexibility inherent in rubber
compounding.

An experienced rubber chemist will be able to analyze the required properties of an application and craft a formula that will
balance the result with the associated cost. The mark of a premier research and development lab is its ability to create a
compound that will meet or exceed the customer's needs while remaining cost-efficient. An excessively optimized compound can
be just as ineffective as a poor-performing product.

2. Rubber Compound Ingredients Breakdown

Understanding key components of a rubber compound is essential for appreciating their roles:

Ingredient Category Effect Explanation

Polymers Determines critical Whether natural or synthetic, polymers define essential properties such
properties as specific chemical resistances, elasticity, and flexibility. Natural rubber,
derived from latex, exhibits high elasticity, while synthetic polymers like
styrene-butadiene rubber (SBR) offer improved wear resistance.

Fillers Enhances mechanical Carbon black and silica, acting as reinforcing agents, provide strength,
performance durability, and wear resistance, enhancing overall mechanical performance.
The specific type and amount of filler significantly impact the compound's
physical properties, such as hardness and abrasion resistance.

Plasticizers Contributes to material Essential for enhancing flexibility and workability, plasticizers ensure rubber
flexibility can be molded or shaped without compromising its inherent properties.
Common plasticizers include oils and resins, contributing to the compound's
processability and elongation.

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 Curing Agents Final Shape Retention Sulfur or peroxides facilitate vulcanization, transforming rubber from a
pliable material into a robust, resilient substance. The vulcanization process
involves cross-linking polymer chains, creating a network that improves
strength, elasticity, and resistance to heat and chemicals.

Accelerators Aids in Final Shape These compounds speed up vulcanization, working with curing agents to
Retention promote efficient cross-linking of polymer chains. Common accelerators
include thiurams and sulfenamides, enhancing the efficiency of the vulcani-
zation process.

Antioxidants Environmental resistance Guardians against oxidation and degradation, antioxidants preserve rubber
integrity, preventing deterioration due to environmental exposure. Hindered
phenols and amines are common antioxidants, extending the service life of
rubber products by inhibiting oxidative processes.

Softeners Aids in processing, and Modifying hardness, softeners enhance processability and impact resistance,
contributes to flexibility contributing to the versatility of rubber. Oils (and some plasticizers) can
serve as softeners, improving the compound's flexibility and low-tempera-
ture performance.

Stabilizers Additional environmen- Defenders against degradation caused by heat, light, and environmental fac-
tal resistances tors, stabilizers ensure long-term stability. Hindered amine light stabilizers
(HALS) and UV absorbers protect against degradation induced by exposure
to sunlight and environmental elements.

Colorants Hue determination Beyond aesthetics, colorants allow for identification or customization, serv-
ing practical and branding purposes. Inorganic pigments and organic dyes
provide color options, ensuring the rubber product meets specific visual and
branding requirements.

Each ingredient plays a crucial role in the composition, contributing uniquely to the final product's properties and performance.

3. Additional Considerations
Integrating a research and development lab with an in-house mixing facility and form production, such as calendering, molding,
extruding, and splicing, is pivotal. The feedback loop allows for both processing and engineering units to improve their products
optimally. This holistic approach ensures consistency and quality throughout the manufacturing process, transforming the rubber
compound into the final product with precision and efficiency.

This intricate combination, guided by the expertise of rubber chemists, defines the dynamic and intricate process of rubber
compounding. The understanding of polymers, fillers, plasticizers, curing agents, accelerators, antioxidants, softeners, stabilizers,
and colorants contributes to the creation of formulations that stand the test of time and performance. The journey from
formulation to production is a testament to the intricacies of rubber production, crafting quality that meets and exceeds industry
standards.

Rubber Mixing
This transformative process initiates the creation of versatile materials that ultimately become functional rubber products. The
process goes beyond the mere mechanical amalgamation of these ingredients; it's a precise science, amplified with artistry, that
determines the intrinsic properties of a rubber compound. Physical characteristics, performance attributes and, ultimately, the
quality of finished rubber products are directly influenced by the mixing process. Understanding the nuances of rubber mixing is
essential for delivering products that consistently meet and exceed both industry standards and customer expectations.

1. The Goal: Ingredient Dispersion

Understanding dispersion and shear in rubber mixing is crucial for achieving consistent and high-quality results. Dispersion refers
to the uniform distribution of all ingredients, which ensures that each component is thoroughly blended. The ultimate goal of
rubber mixing is to achieve a high level of dispersion as poor ingredient distribution can lead to failed physical properties and a
lack of desired characteristics. Lumps, pinholes or a bumpy, uneven textured surface on finished parts can be a key indicator of
poor dispersion. This can result in sub-standard wear and propagate fissures and tears.

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Shear describes the force applied to the rubber compound to drive dispersion, with higher shear rates leading to better
dispersion. By controlling shear forces, rubber manufacturers can optimize the mixing process to develop the desired properties in
their products.

There are four key measurement parameters that play a significant role in achieving effective dispersion:
• RPM or revolutions per minute indicates the speed of mixing equipment rotation
• Amps measure the electrical current drawn by the mixer and correlates to the workload of the mixer
• Temperature is monitored to precisely blend ingredients and prevent scorching or premature curing
• Time is fundamental to control the duration of mixing required to reach homogeneity

2. Two Methods of Rubber Compound Mixing

There are two primary methods for mixing rubber compounds: mixing with an open-mill or utilizing an internal mixer. While open-
mill mixing offers simplicity and provides for small batch sizes, internal mixer mixing provides enhanced control and efficiency
gains through speed and larger batch sizes.

Open-Mill Mixing
Open-mill mixing occurs on a two-roll mill, a foundational method facilitating
efficient blending and dispersion. The shear rate is primarily controlled by
adjusting the speed of and the nip or space between the mill rolls. However,
achieving uniform dispersion may be challenging due to the limited control over
mixing parameters compared to internal mixer mixing.

Open-mill mixing's strengths lie in its simplicity, low equipment cost and ability
to mix small batches. However, it comes with limitations regarding precision and
control over mixing parameters. It typically allows for the measurement of only
two of the four key mixing parameters: RPMs and time. Moreover, it is a slower
process with each batch requiring 30-45 minutes to blend properly. Achieving
nuanced formulations, particularly those with specialized requirements, may
present challenges.

Internal Mixer Mixing

Internal mixers are specialized machinery, offering heightened control over
mixing parameters which provides more consistent results. This method ensures
a precise blending process that enhances the overall quality of the rubber
compound.

Each of the four key mixing parameters are precisely optimized as well as a
host of additional controls and monitoring functions. Operators can perfect the
shear rate of the mixer rotors against the walls of the mixing chamber to ensure
thorough dispersion of ingredients, leading to a uniform and consistent rubber
compound.

Internal mixer mixing is a complex process that offers improved control and
efficiency along with a higher investment in machinery. The benefits include a
precise blending process, significant speed improvement of a mere 5-7 minute
cycle time per batch, and command over mixing parameters. This level of control
is integral for high performance compounds designed for applications requiring
critical performance and specific requirements.

3. Steps in the Rubber Mixing Process

1. Introduction of the Polymer

The rubber mixing process commences with the introduction of the polymer, the fundamental building block of rubber
compounding. The choice of polymer establishes the foundation for the material’s performance and dictates its inherent
characteristics. See warco.com/polymers to learn about each polymer’s properties.

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 2. Sequenced Addition of Ingredients
While the polymer typically has the largest impact on the characteristics, additional ingredients are included to determine
features such as curing and processing performance, cost consideration, hue designation, anti-degradation, and much more.
This includes fillers, plasticizers, curing agents, accelerators, etc. See our previous article, warco.com/the-art-of-rubber-
compounding for a general breakdown of each ingredient type’s effect.

3. Achieving Homogeneity in the Compound

The ultimate goal is achieving thorough homogeneity. A seamless blend of all ingredients creates a uniform and consistent
rubber compound. A homogeneous rubber compound ensures uniform characteristics throughout production processing,
curing and the final application of finished rubber goods. This is achieved by carefully moderating the four key mixing
parameters according to the compound requirements while carefully considering chemical interactions of ingredients. Each
rubber compound performs and reacts differently to the process and therefore requires an experienced operator execute on
each compound’s detailed and exact mixing procedure to create the ideal mix.

4. Key Takeaways
The key to manufacturing quality rubber products begins with precise control over compound mixing parameters coupled
with a deep understanding of the materials and their interaction with each other through the process. Our technical
managers leverage their expertise in chemistry, mixing parameters, compound ingredient reactions, form production process
and cure methods, and more to establish the ideal procedure of temperature, time, ingredient sequences, and shear rate.

An effective rubber manufacturer must be well equipped with testing equipment to measure the unique parameters of a
compound prior to and after the form/cure production process. This feedback loop allows for a higher standard of quality and
consistent, repeatable, and reliable results.

WARCO develops proprietary formulations, mixes compounds, and produces your sheet, molded and extruded rubber products in
a single facility. Our expertise spans beyond our 114-year history providing quality rubber products to American industry and the
global supply chain. Our wealth of experience, unique flexibility and integration, and tight control over each step of production
are among the key contributors to how WARCO became America’s Choice for Quality Rubber.

If you have any questions for us, please reach out by phone, 714-532-3355, by email, sales@warco.com or learn more at
warco.com.

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