Glass Fibre Manufacturing and Sizing Stability

Sizing development for composite and fiber production is highly specialized and complex. Sizing development is a complex process. Understanding most of the process, property and chemical structure-property relationships involved in sizing performance for composite and fiber production is difficult. Michelman however, has a deep knowledge due to years of experience and countless trial and error experiments. This puts them in the ideal place to explain it to the advanced composites materials industry, and bring their sizing expertise forward for the benefit of others. Sizing therefore, helping composites rapidly change the way we drive, generate energy and transport goods. It is also putting forward solutions for problems involving the contradiction between achieving maximum strength or minimal weight. This article will discuss the role sizing plays in glass –reinforced composites.

What is Sizing?

A sizing (the word “finish” is sometimes used, which is derived from the textile industry) is a combination of a various chemicals, mostly diluted in water, which fabric and fiber producers use to coat their fiber. In several instances, the sizing can be regarded as a coating on the fiber, and in many cases, it is what typically differentiates the fibers made by separate manufacturers. Each manufacturer has developed their own method of sizing and how to apply it to the fiber to suit the needs of the targeted application. Michelman has developed a range of formulations involving binders and film formers.

It is critical in the formulation of a sizing for particular application to understand what exactly it will be used for. Michelman has established and grown this expertise over many years and is a leading global supplier of these materials.

Sizing Application in the Fiberglass Manufacturing Process

One of the key features that fiberglass manufacturers are looking for in the procurement of raw materials is sizing’s stability. Figure 1 shows how as part of the manufacturing process, glass fibers are cooled with water and air spays once they descend from the hot bushings. Once they are cool, the fiber coating is applied with a cylindrical kiss roller that holds the sizing from the application roller.

Fiberglass manufacture: bushing

Figure 1. Fiberglass manufacture: bushing

The graphite roller sits in a tray that holds the sizing, and constantly rotates as the fiber moves to apply a uniform coating of thin sizing film. It is vital that this rotation speed, sizing solids content and the sizing viscosity is controlled so that the optimal sizing amount is delivered. Once coated, the fibers are either segmented while wet or wound wet then dried through a hot oven or using radio frequency electromagnetic radiation. This is a continuous manufacturing process and the line speeds can be extremely fast.

The sizing mixture is prepared on-site and is stored in a holding tank where it is constantly moved to prevent settling. Depending on the number of bushings running with a specific sizing, the sizing can last anywhere from 1-5 days.

As sizing is applied and volume reduces, a new sizing batch is prepared and mixed into the holding tank with the old one. As well as this, the sizing is re-circulated constantly between the applicator tray and the holding tank. With the amount of shear applied to the sizing over this whole process so it is expected to have no or little foam.

Usually the holding tank is kept at a low temperature to prevent the sizing from breaking down, which can be down to increasing particle size and viscosity, and/or grit beginning to appear on the applicator roller.

In order to achieve smooth, reliable and consistent production, the sizing must stay consistent to as when it was initially prepared. Hence, during fiberization, the sizing is monitored by checking the pH, viscosity, solid content etc. Efficiency losses can take place if the sizing quality is degraded as shown in the following chart:

Table 1. Effect of Sizing Variation on Fiberglass Production

Sizing problem Effect on Fiberglass
Increase in particle size Increase in LOI (Loss on ignition*)
Appearance of grit More breaks of fiber during application
pH changes Increase or decrease in the sizing pick up
Foam Fuzz, changes in LOI, uneven coating

*Amount of organics on the fiberglass after it has dried.

Finding the Balanced Recipe for a Stable Sizing Formulation

Sizing stability is the ability of a formulation stay in a homogeneous state from its first mixing to the when it’s coated onto the fiber. Glass fiber sizing at its most basic is water and a coupling agent. As the glass is inorganic in nature, coupling agents are used to bond the resin matrix (organic) to the glass surface (inorganic).

Organo-silanes are small, yet very effective, molecules and are the preferred coupling agents used in the fiberglass industry. However, if only organo-silanes are used, the sizing will embrittle the fiber and make it almost impossible to handle in the compounding process. Due to this, sizing is usually a mixture of several ingredients that serve a specific function in the mixture:

  • Coupling agents,
  • Film formers,
  • Additives, and
  • Water

For thermoplastic reinforcements, γ-aminopropyltriethoxysilane, NH2-(CH2)3-Si(OCH2CH3)3, as shown in Figure 2, is a workhorse in the industry, and is used about 95% of the time. Other factors including pricing, availability and performance also explain why this material is chosen most often.

Other amino-silanes used in thermoplastic resin fiberglass production include:

  • N-β-(Aminoethyl)-γ-aminopropyltrimethoxysilane NH2-(CH2)2-NH-(CH2)3-Si(OCH3)3(Figure 3) and
  • The triaminofunctional silane NH2-(CH2)2-NH-(CH2)2-NH-(CH2)3-Si(OCH3)3 (Figure 4).

γ-Aminopropyltriethoxysilane NH2-(CH2)3-Si(OCH2CH3)3

Figure 2. γ-Aminopropyltriethoxysilane NH2-(CH2)3-Si(OCH2CH3)3

N-β-(Aminoethyl)-γ-aminopropyltrimethoxysilane NH2(CH2)2NH(CH2)3Si(OCH3)3

Figure 3. N-β-(Aminoethyl)-γ-aminopropyltrimethoxysilane NH2(CH2)2NH(CH2)3Si(OCH3)3

Triaminofunctional silane NH2-(CH2)2-NH-(CH2)2-NH-(CH2)3-Si(OCH3)3

Figure 4. Triaminofunctional silane NH2-(CH2)2-NH-(CH2)2-NH-(CH2)3-Si(OCH3)3

Film Former

A film former protects the fiber is during processing and manufacturing. During fiber manufacture, every contact point is a possible fiber break point. These contact points are an unavoidable part of the process, so a film former is used for fiber protection so that it remains undamaged.

Contact points can arise from drying, winding and packing of the fiber, with even more contact point arising as weavers and compounders are processing the fiber. Protection of the dry fiber is an important consideration during these processes as well.

The overall particle size of the solids in the sizing mixture is dependent on which film former is used. On the fiber surface, the film former plays a critical role in determining the final composite properties such as offering improved adhesion between the resin matrix and the fiber which in turn affects the mechanical properties, chemical or water resistance, thermal stability, etc.

There are many waterborne film formers available commercially, such as epoxy, polyurethane, polypropylene, acrylic, polyethylene, polyamide, polycarbonate, polyvinyl acetate, etc. Michelman’s Fiber Sizing & Composites business unit develops film formers specifically for this industry as shown in Figure 5. With a broad portfolio of film formers, it is committed to serving the needs of this industry. These products are unique among Michelman’s range as they are specifically designed film formers that address the challenges unique to the fiberglass manufacturing process.

Michelman Film Former Selection Guide

Figure 5. Michelman Film Former Selection Guide


There are several reasons additives may be used such as stabilizing the sizing, improving pick up, providing fiber lubricity, preventing composite discoloration, dissipating static during processing, improving fiber process-ability, preventing wear and tear of the equipment, and keeping the sizing from breaking down. Yet they only account for a small percentage of the total sizing formulation (5%-10%).

Very few additives are made with just the fiberglass industry in mind, with most coming from the paint, coating, textile, and polymer industries. Additives full under classifications such as surfactants, lubricants, anti-stats, antioxidants, etc. As is the case with its film-forming products, Michelman has developed a family of lubricants dedicated to fiberglass sizing, as shown in Table2 below.

Table 2. Properties of Michelman Sizing Lubricants

Michelman lubricants % Non-volatiles pH Recommended pH Melting point (DSC, 10 °C/min)
Hydrolube® 480 40 <10.5 1.0-12.0 50– 100 °C
Hydrolube® 723 32 7.0-10.5 2.0-12.0 54 °C
Hydrolube® 763 10 4.0-10.0 2.0-12.0 63 °C

Testing Sizing for an Optimized Production of Glass Fiber

With no standard test for sizing stability, to account for stability in fiberglass sizing the film former or additive has to be stable in water in the presence of an organo-silane. On top of this, all the sizing components must stay in their original form throughout the lifetime of the sizing (1-5 days, see above). Due to the shear and temperature changes during the delivery process, the particles that make up the dispersion must be strong.

Here are some general statements based on a review of several sizing formulation patents:

  • Organo-silanes are used at levels ranging from 1%-25% of the total solids;
  • Film former levels range from 75%-99% of the total solids;
  • Additives are used at less than 5% of the total solids; and
  • The total sizing solids content varies anywhere from 5%-12%.

Michelman has developed a method to test its own products to check the robustness in fiberglass sizing and to continually improve the stability in its products. As new dispersions are developed, stability testing helps to identify the optimal surfactant system and makes it quicker and easier to identify a final, organo-silane stable formulation.

Figure 6 presents a mixture containing a film former, water and varying concentrations of silane, with the level of silane increasing from left to right. As the film former is usually a dispersion, the particles can aggregate in the presence of other materials, and in this case, organo-silanes. Tube 1 shows the emulsion coexisting with the silane without breaking down. Compare this to tube 5, which contained the highest level of silane, where the emulsion has totally broken down.

Michelman screening test for sizing formulation stability

Figure 6. Michelman screening test for sizing formulation stability


Sizing is a complex mixture with multiple ingredients, so it is of paramount importance that the selection of these ingredients complement each other in the fiber manufacturing process and meets the end-use application requirements. Sizing stability is critical to the glass fiber manufacturing process, in fact, glass fiber manufacturing cannot use an unstable film former.

It is important that any potential sizings are pre-screened for stability to isolate any ones that do not meet manufacturing guidelines and save the time and resources for the fiber manufacturer. As well as sizing stability, the characteristics of the sizing formulation – particle size, pH, rheology and other attributes – all can affect the performance of the fiber and its manufacturing process.

Finally, the performance requirements of the composite application – strength, stiffness, chemical resistance, etc. – all directly influence the selection of ingredients. Michelman is a noted leader in this industry, and you are encouraged to contact them if you have any questions about any of these processes.

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

For more information on this source, please visit Michelman.


  1. Eman Omar Eman Omar Egypt says:

    I'm interested in possibly starting in making research study for development the manufacturing and sizing of glass fiber. Where I can buy lab-scale equipment??


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