Components of Fiber Sizing Formulation

Steve Basseti

Fiber sizing has an important impact on the final properties of composite parts; this is according to the Michelman Industrial Manufacturing’s marketing director, Steve Basseti, Gilles LeMoigne, the company’s fiber and composites industry manager and Nagesh Potluri, the group technology director.

Can you share a little information about Michelman and its activities?

Steven Bassetti: Founded in 1949, Michelman has gained immense popularity because of its capabilities in the area of emulsification and polymer dispersion technologies. The company acquired Hydrosize Technologies in 2008 due to its early achievements with polypropylene sizing applications.

Michelman sustained innovations around the materials and process capabilities since it has acquired the Hydrosize brand. We produce a diverse line of fiber sizing and surface treatments that are utilized by most composite manufacturers and fiber producers to create more lightweight, stronger and more durable composite parts.

We manufacture a versatile line of fiber sizing and surface treatments that are used by fiber producers and composite manufacturers to produce stronger, lighter and more robust composite parts.

The firm’s materials are used in the production and manufacture of natural fibers, glass basalt carbon and fabrics created from these fibers. In the long run, we have evolved to experts in terms of interface adhesion because of our know-hows in technology and a varied product range that are augmented for a diverse array of thermoset, thermoplastic and technical textile fabric applications.

Michelman is dedicated to serve the industry and embark on a multi-year investment strategy that will enhance its expertise and capabilities to further improve its sizing and composite development.

What is fiber sizing?

SB: Fiber sizing is a thin homogenous coating applied on the surface of the fiber during production. This thin coating works by protecting the fiber when it’s being manufactured. Aside from that, it also protects the fiber during post-processing operations including spreading, handling, compounding, weaving and molding.

Fiber sizing also provides a bond to the matrix polymer used to produce the technical or composite textile. Usually, the fiber manufacturer produces the final sizing formulation, including some ingredients such as defoamers, coupling agents, lubricants, film formers and other additives.

Gilles LeMoigne

Why is it important?

Gilles LeMoigne: Sizing makes the most of the process ability of carbon fiber. Despite the fact that carbon fiber composites only need a small percentage of sizing – about 0.5 to 5 percent of its weight, it can perform various tasks. These include protecting the fiber during production, as well as aid in the processing of the fiber into intermediary forms like braids, fabrics, and thermoplastic organo sheets.

The company’s technology allows the customers to optimize the interfacial adhesion between fibers and polymers by modifying the surface chemistry of reinforcement fibers to the matrix resin chemistry.

What are the components of a fiber sizing formulation?

Nagesh Potluri: The fiber sizing formulations contain water, film formers in dispersed, dissolved or emulsified forms, coupling agents for basalt fiber and glass, and a wide array of modifiers or additives, including plasticizers, anti-static agents, surfactants, rheology modifiers and antifoams. When these components are mixed, they are delivered to the fiber as the final sizing formulation.

Nagesh Potluri

On the surface of the fiber, the film former plays a pivotal role in identifying the properties of the final composite, like the improved adhesion between the matrix and the fiber. Also, it determines the mechanical properties as it enables effective load transfer between the fiber and matrix, along with water or chemical resistance and thermal stability. Today, there are hundreds of film former dispersions that can be bought commercially, but only a few are well-matched with other sizing components.

As just a coating on the fibers, what impact can sizing have on those employed for reinforcing composites, if they’re encased in resins?

NP: The key to improve and enhance the composite properties is the compatibility of the sizing and resin matrix. Sizing has a noteworthy impact on the interface properties of the composite matrix and fiber surface. Also, it affects the processing and manufacture of the fiber.

When there is right sizing, the matrix and fiber will become compatible. By monitoring the surface chemistry of the sized fiber, the compatibility is attained. Moreover, when there is proper design, there is improved interface that can help match the elongation at break between the matrix and the fiber.

When is the sizing applied in the production of carbon fiber?

GLM: There are six parts in terms of carbon fiber production (as shown in Figure 1). Sizing is usually applied near the end of the process. Each manufacturer of fiber improves its own sizing knowledge and applies it to the fiber to conform to the needs of the targeted usage.

We appreciate what role sizing plays in the production of composites and fiber. In fact, we work with all the constituents of the value chain to come up with sizing for specific resin attributes. Also, to formulate sizing for particular properties preferred in composites.

What effect can the stability of the sizing have on carbon fiber?

GLM: When referring to sizing stability, it’s the ability of the formulation to remain in a state of homogeneity from the moment it’s mixed to the time it is transported onto the fiber. Stability is one essential characteristic that carbon fiber produces, take into consideration when it comes to choosing raw materials for their formulations or preparations of sizing. In fact, there are about 4 major effects of sizing diversity on fiber manufacturing (as shown in Table 1).

Effect of Sizing Variation on Fiber Production
Sizing Problem Effect on Fiber
changes in particle size changes in LOI (Loss on Ignition*)
short shelf-life increase in manufacturing downtime
pH changes increase or decrease of sizing pick-up on fiber
foam fuzz, changes in LOI, uneven coverage

*Amount of organics on the dried fiber.


What are the benefits of sizing on a) the fiber itself, and b) the finished composite, in respect of both manufacturing, and further processing into composites?

SB: Regarding the production and further processing of fiber, the changes seen in sizing film formers can be advantageous to both the physical characteristics and the furnished composite properties (as shown in Table 2). Sizing is beneficial when it comes to carbon fiber production particularly in protecting the freshly formed fiber surface from abrasion. It also prevents fiber breakage at contact points and helps preserve the chopped bundles of fiber.

GLM: There are many ways on how sizing aids in the protection of carbon fiber during the processing and handling. It does not only provide integrity by keeping the chopped bundles together, but it also aids in chopping since it heightens the chopper’s blade life. At the same time, it decreases fuzz generation. Regarding continuous fiber, both the weaving and unwinding processes are supported since sizing bonds the fiber strands together while giving flexibility and lubricity. These help in fiber spreading and avoiding breakage or damage.

NP: The contribution in sizing’s contribution in making handling easy cannot be overestimated. The weavers have worked for fiber manufacturing companies for decades already in order to develop the sizing that is effective. Fibers are considered effective when they are passed through a liquid bath and form a coating on the surface of the fiber, which successfully lubricates the fiber to augment production speed and assist in the weaving process in many ways that create more cost-efficient and high-quality carbon fiber fabrics.

Benefits of Fiber Sizing
Fiber Physical Characteristics Composite Properties
discoloration thermal stability
solubility degradation
resin wetting surface finish
ease of chopping water resistance
reduction of static chemical resistance
FDA compliance color
reduction of fuzz fiber length
bundle integrity adhesion


How does sizing further enhance the mechanical and chemical properties of composites?

SB: Sizing improves a wide array of chemical and mechanical properties in composites because of the pivotal role it plays at the interface of the matrix and fiber. However, failure modes for composites can happen at the interface, in the fiber or matrix. The application and its requirements can influence the possible failure mode and the part that sizing can play in alleviating the failure.

The advantages of sizing to composites are:

  • Compressive strength
  • Corrosion resistance
  • Thermal and hydrolytic stability
  • Electrical conductivity
  • Impact strength
  • Fatigue performance
  • Heat and oil resistance
  • Tensile and flexural strengths

What are the key considerations for choosing the correct sizing for use in composite reinforcements?

NP: The type of fiber, fiber processing, resin and final composite application are essential considerations to take in selecting sizing.

Thermoplastic or thermoset resins require various chemistries to create an efficient interface between the matrix and fiber surface. If it’s not compatible with one thermoplastic resin or thermosetting might be incompatible with another. When it comes to thermoset resins, the interface has an opportunity to cross link with the film formers on the fibers during the curing process.

However, special chemistries require being delivered through sizing to prompt the cross linking between film formers and thermoplastic resins. Apart from conventional chemistries, Michelman boasts unique polyamide, polyurethane and polyimide sizing chemistries to efficiently cross-link fiber and thermoplastic resins via the film former.

Michelman has dynamic development programs that aim to develop products for the high temperature processable engineered thermoplastic markets that are usually underserved from a sizing position.

GLM: Various sizing are required between chopped and continuous fibers. For instance, chopped fiber should contain a tremendously good bundle integrity or it will be hard for the fiber to be fed into the compounding machine. Yet, continuous fiber applications can use a sizing that can promote a better filament spreading for an easier wet-out.

The necessities for the final application of the composite part are crucial in sizing selection. Sizing chemistry plays a pivotal role in the mechanical properties of a composite. These include tensile strength, impact resistance, fatigue resistance, and mechanical and chemical properties like hydrolysis, heat, oil-resistance and corrosion.

Michelman recently introduced a number of new products, including Hydrosize Link. What is it and what are its applications?

SB: When we talk about hydrosize link, it’s described as an improved reactive sizing that boosts the chemical resistance of polyamide composites. It’s developed for long-lasting performance and durability, including exposure to harsh fuels which allows car makers to cost-effectively conform to emissions regulations and improve fuel efficiency.

(Figure 2 shows how Hydrosize Link provides superior resistance to glycol fluids used in coolant.)

Theoretically, hydrosize link is a series of polyurethane film formers developed to offer improved and enhanced reactivity with the polyamide matrix resin.

NP: During the fiber manufacturing process, these high performance and quality sizing produces are added. They are also activated during the compounding process to boost the interfacial adhesion between the fiber and polymer matrix. The enhanced adhesion gives good resistance to the glycol fluids utilized in coolant systems and provides an avenue for car manufacturers to develop efficient components that perform to the exact specifications needed.

This information has been sourced, reviewed and adapted from materials provided by Michelman.

For more information on this source, please visit Michelman.


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