Some insights into proprietary chemicals that can customize fiber/resin interface properties. #sizing #adhesives #paek

Figure 1: Proprietary, but critical to performance. Sizing prevents the fibers from fraying/breaking during initial manufacturing and downstream processing (such as weaving), and ensures bonding at the critical fiber/resin interface. Source: Owens Corning

Figure 2: Unique proprietary formula. The chemist mixes sizing in a clean room environment. Sizing chemistry is one of the highly protected trade secrets, partly because it is one of the most important factors that distinguish one fiber product from another. Source: Michelman

Figure 3: Dimensions tailored to the task at hand. Car interior instrument panel support beams, armrest frames and center console brackets are one of many components made of fiberglass/polypropylene, and their dimensions are tailored, customized and optimized in a way that suits the end-use application. Source: Michelman / Photo: Thinkstock (Photographer/Maudib)

Figure 4: Optimizing the size of fiber-reinforced thermoplastics. This car front-end module used to install car radiators and other components is made using the LFT process, using glass fiber and Michelman (Cincinnati, Ohio, USA) custom-made Hydrosize sizing for the polypropylene matrix. Source: Michelman

Figure 5: The experiment uses SiO2 as an additive. Tests conducted by researchers at Evonik (Essen, Germany) have shown that adding nano-silica to the glass fiber sizing formulation can significantly improve the performance of wind turbine blades. Source: Evonik Industries

In the composites industry, fiber is an important topic. Fiber sizing is rarely mentioned. However, it is difficult to exaggerate the importance of size. Without it, the fibers will quickly begin to wear during handling and processing (see Figure 1, left), and will hinder adhesion to the matrix resin. In addition, sizing can also improve mechanical properties, such as tensile strength and impact resistance, fatigue resistance and chemical resistance, and improve thermal and hydrolytic stability.

The largest sizing consumer is the glass fiber manufacturer. According to Professor James L. Thomason of the University of Strathclyde (Glasgow, Scotland, UK), the global annual output of glass fiber composites is close to 9.07 million tons. 95%. 

PPG Industries Inc. (Shelby, North Carolina, USA) is one of the largest fiberglass manufacturers in the world. It uses silicon dioxide (SiO2) sand and other selected raw materials to be eutecticized in a 1450°C furnace, and is guided to shape glass fibers through a molding sleeve (click "Glass Fiber Manufacturing" under "Editor's Choice" to learn more Information) upper right). The fiber sizing is applied immediately after the filament leaves the sleeve. “We apply a coating of about 100 nm thick on a cylindrical surface with a diameter of about 10-20μ and move it at a speed close to 100 mph [160 kph],” explains James C. Watson, deputy director of the Glass Fiber Science and Research Department Say. For PPG Industries, "We do this work 24 hours a day, 7 days a week, 365 days a year. We never stop."

"It's a bit challenging," he quipped, noting that the sizing agent must remain on the fiber after application, and in all subsequent stages of the glass manufacturing process, the customer's part manufacturing process, and the completed part life cycle. He added that in terms of the total amount of composite materials, sizing accounts for only 0.2-0.3% of the final product, but it "has an amazing effect on productivity and part performance."

Sizing describes the coating itself and the coating process separately. Whittington's "Plastic Dictionary" (third edition, CRC Press, 1993) defines the latter as "applying a material to a surface to fill in pores to smooth it and reduce the absorption of subsequently applied adhesives or coatings; or Modify the surface in other ways."

Today, the primary goal of sizing chemists (see Figure 2, left) is to protect the fibers by reducing friction during processing and prevent fiber breakage during part manufacturing. Second, they provide a compatible interface between the fibers in the composite part and the matrix resin. Specifically, sizing can transfer stress across the fiber/resin interface.

"Sizing controls the interface between glass and resin. Therefore, optimizing sizing chemicals can provide you with the best interface performance between glass and resin. Good interface performance can improve mechanical properties and processing performance," Glass Fiber Manufacturing Explains Christopher Skinner, product director of the strategic marketing team at Owens Corning (Toledo, Ohio, USA).

Glass fiber sizing agents mainly include film-forming agents, which are usually chemically similar to the expected matrix resin, and coupling agents, which are almost always organic silane bases. The chemical mixture is complex, so that inorganic glass fibers can be well combined with organic resins. matrix. The silane coupling agent acts as a primer, enabling the film former to adhere to the fiber and to the polymer matrix.

Watson pointed out that silane is the main force of glass fiber sizing chemists. "But as we move towards higher temperatures, higher durability, longer service life, better fatigue performance and other improved properties, we must ask ourselves whether there are better organosilane molecules available to achieve These properties-even if they are always more expensive."

Steve Bassetti, global marketing manager for fibers and composites, said that today, there is no universal sizing agent—there is no one-size-fits-all formula for fiber sizing agent supplier Michelman (Cincinnati, Ohio, USA) . "There are several key variables that need to be understood in the sizing selection process," he said, and expanded his theme: "Obviously, [one is] the resin into which the fiber will be put, whether it is polyester, epoxy, or polypropylene. , Nylon, PAEK or others."

For example, Fausto Pellegrini, key account manager and team leader of Aliancys AG (formerly DSM Composites, Schaffhausen, Switzerland) said that Aliancys is studying sizing agents with excellent wettability and compatibility with new styrene-free resins. . "Usually," he explained, "when used with new styrene-free resins based on methacrylate solvents and derivatives, sizing chemicals containing styrene thermoset resins perform poorly."

Second, Bassetti said, it is the expected process, including whether the sizing is used for chopped or continuous fibers and molding methods, such as compression molded sheet molding compound (SMC) or filament winding.

Watson of PPG said: "An important trend in the past 5 or 10 years is that in addition to the traditional E glass, different types of glass composition have been greatly expanded, extending to more corrosion resistant ECR glass and high modulus glass." Therefore Sizing products are also increasingly driven by glass composition changes and subsequent fiber surface changes to make the sizing compatible with corrosion-resistant, low-boron or boron-free glass, such as high-modulus glass blades for wind turbines, He said.

Pellegrini said Aliancys is developing sizing agents for specific in-line wetting processes (such as pultrusion and filament winding) to reduce part production time and process energy consumption. Last but not least, the chemist must consider the performance of the part and the environment in which the part will be installed—car interiors, engine hoods or fuel system components, electronic laptop components, fan blades, or one of countless other market applications . This includes the challenges faced by chemists who adjust the size of parts. Aliancys' Pellegrini says that sizing agents based on low molecular weight epoxy chemistry have been successfully used for wind turbine blades and other direct roving applications for many years. However, wind blades are getting longer and longer to increase the power output of wind turbines, emphasizing the need for durability, stiffness, and fatigue resistance of fiberglass blades.

Skinner pointed out that blade manufacturers have invested "incredible money" in "tools," such as 75m blades. Then "They put more than 40 tons of glass into the mold and infused it with resin." With so much capital in fixed assets, the ability to quickly manufacture parts is crucial. "Therefore, the ability of sizing chemicals to respond to the ever-expanding scale of applications is a key factor in sizing innovation," Skinner said. Here, the fiber/resin combination is the key. "So we are promoting interface science to improve the performance of our products."

Bassetti explained that the industry requires improved fiber properties, which will require customization, customization and optimization of specific sizing agents in a way suitable for end-use applications, and pointed out that for Michelman’s sizing product line, including its registered flagship product Hydrosize As the advancement of resin and fiber technology reaches maturity, the fiber interface becomes more and more important. "Today's research is aimed at specific applications," Bassetti pointed out, citing examples that may be looking for a low-VOC polypropylene supplier in car interiors (see Figure 3), or may require higher or better hydrolysis resistance nylon Supplier ethylene glycol resistance.

Bassetti summed up this trend in one word: specialization.

Pellegrini built a trend of specialization based on global market pressures. The demand for glass fiber will continue to be strong, but fierce price competition from Asian fiber suppliers has become a factor challenging incumbent glass fiber manufacturers to focus on reducing costs by improving efficiency and productivity.

“The annual growth rate of composite materials is usually 4-5%,” OC’s Skinner agreed, based on well-documented composite materials replacing traditional wood and metal to reduce weight and/or prevent corrosion, but he also pointed out what he called Professional characteristics-specific durability or aesthetics, acoustics or weather resistance requirements for building materials.

Skinner added that the EU's REACH legislation aims to strengthen humans and environmental protection from the risks posed by certain chemicals, and is also promoting the development of dimensions. This is not only about low volatile organic compounds in the sizing process, but also about the highly specialized chemicals used for sizing to produce specific effects. "The REACH regulation has played a role in specifying approved chemicals," Skinner pointed out. Therefore, a major challenge for sizing chemists is to achieve the desired results in compliance with the legislative environment. In other words, "How do we continue to innovate and respond to corporate social responsibility?"

Pellegrini pointed out that Aliancys' R&D work on VOC-free sizing formulations to reduce environmental impact includes sizing chemicals that comply with the new European regulations on food contact parts.

"We are seeing more and more demand from the automotive industry," said Gilles LeMoigne, Michelman's global industry manager for fibers and composites, because automakers' chemists are also looking for solutions. "Michelman's goal is dimensional design to help the automotive industry meet the latest regulations for composite manufacturing." These goals include low volatile organic compound (VOC) or no volatile organic compound (VOC) in its sizing, as well as meeting Sizing targets for specific targets for impact resistance or tensile strength.

At the forefront of the trend of custom-designed dimensions are thermoplastic composites. Bassetti insists that any composite, and a thermoplastic in particular, is under-optimized when a general purpose sizing is selected. "Similarly, our business is to professionalize the interface and improve composite performance​​," he said.

But he warned that the latest thermoplastic sizing agents are still under development and verification, and good products may not be available on the market. In this field, as well as in the field of thermoset plastics, he reminded us that Michelman is working on specific sizing for different applications: long-fiber thermoplastics (LFT) using polyamide (nylon) and polyolefin-mainly polypropylene (PP) )-Designed, for example, on car seat backs, load-bearing floors, door modules and other interior components.

Skinner sees a general trend in resin selection from polyester and vinyl ester to polyurethane (PUR), a class of polymers with a wide range of properties, which can be thermoset or thermoplastic. "I expect the ability to effectively interact with the PUR matrix will become more important in the future," Skinner said.

A real dimensional "hot spot"-vividly and literally-is the application under the hood of a car. High temperature and corrosive liquids such as glycol (antifreeze) and other thermal fluids are proposed for the design of glass fibers. Strict requirements," PPG's Watson said. "Dimensional design can improve performance by several orders of magnitude, especially in terms of high temperature and hydrolysis." The resins most suitable for this extreme environment are usually thermoplastic. The main force here is nylon (polyamide), PP and thermoplastic polyester varieties, polyether ether ketone (PEEK), polyphenylene sulfide (PPS) and polyaryl ether ketone (PAEK) resins are limited in number.


Bassetti added that emerging technologies also need to customize and improve the size to get the best interface features. For example, a new method of using continuous fibers or higher aspect ratios to add strength to composite materials-requires thermoplastic prepregs or organic sheets (strong, lightweight continuous fiber reinforced thermoplastic structural parts) and thermoplastic resin transfer molding ( RTM) — creates new opportunities for optimizing sizing and fiber/substrate interface, Bassetti said. Michelman is actively performing size upgrades to meet these improved requirements (for example, see Figure 4).

The latest sizing technology

Silanes and other sizing ingredients, such as dispersants and emulsifiers, are provided by various divisions of the chemical manufacturer Evonik (Essen, Germany). Evonik applies part of its research to new sizing systems and compositions. The ingredient that best distinguishes one fiber manufacturer's product from another's comparable fibers, sizing is highly proprietary (see the "Editor's Choice" in the upper right corner for more information). "Fiber sizing is a secret of fiber manufacturers," said Stephan Sprenger, senior market development manager for composites and lightweight structures in Evonik's business line interface and performance department. Therefore, when Sprenger revealed to CW that his team was investigating possible improvements when adding nano-silica (SiO2) particles to sizes less than 1 wt/%, this was a notable exception.

"When a cyclic force is applied to a fiber-reinforced composite material or a sudden impact occurs, microcracks will form in the resin matrix. Over time, the cracks will pass through the matrix and propagate along the fibers," Sprenger said. The test concluded that the toughness and fatigue properties of glass or carbon fiber composites can be significantly improved by adding "a small amount of nano-silica-containing epoxy resin aqueous emulsion" to the sizing agent, Sprenger said.

In tests conducted using continuous glass fiber and continuous carbon fiber in unidirectional fiber-reinforced epoxy resin, the fatigue performance was improved by 30-40%. Sprenger points out that, for example, this will significantly improve the performance of rotor blades used for wind energy conversion (see Figure 5 below). For more information on carbon fiber sizing, please refer to the side story at the end of this article, or click on the heading "Carbon Fiber Sizing" under "Editor's Choice".

An aqueous emulsion called Nanopox has been tested with an epoxy resin matrix as a sizing component, and Sprenger and his team are studying its use with unsaturated polyester and vinyl ester resin matrix. Sprenger believes that Nanopox is a real breakthrough for sizing manufacturers at a small additional cost. In addition, it is considered suitable for any thermosetting resin, but Sprenger said it may not be suitable for thermoplastic resins.

The same nano-silica particles can also be used to modify the matrix resin to achieve greater improvements in automotive composite leaf springs and other components. But he added, “Modifying the matrix comes at a price, and it may affect the speed of the manufacturing process accordingly.” Sprenger emphasized that adding it to the fiber sizing agent is a very cost-effective method that can be used in wind turbines. Improve the performance of composite materials in cost-critical applications such as rotor blades or mass-produced automotive parts.

Over the years, as composite materials have taken a place in automobiles, airplanes, construction, pipes, and other markets that were very satisfied with concrete, wood or metal at the time, sizing has played a key role in the successful replacement of fibers, but often Obscured. Reinforced polymer for traditional materials. But even in the most successful cases, designers, manufacturers, and end users want far more than substitutes. They need fiber-reinforced polymers that have higher strength, toughness, and durability than traditional materials. They want faster productivity. Everything is at a lower cost.

Can fiber manufacturers, sizingists and formulators really achieve the performance and productivity upgrades that customers require? They are very aware of this need, and of course they are ready to meet the challenge. Sizing and fiber manufacturers are coordinating with their customers to understand their needs. They fed this information back to their scientists. They focused their chemistry on fine-tuning sizing formulations, making specialization a reality, and developing customized, customized and optimized sizing agents to meet industry needs.

Carbon fiber sizing 

Alan Riley, general manager of the Nordic Carbon Fiber Department of Toray Carbon Fiber Europe (London, UK), said that carbon fiber sizing has two main functions, both of which are similar to glass fiber sizing. One of the main goals is to protect the fiber, prevent individual filaments from breaking and improve the handling of very fine carbon filaments, usually 5 to 7 microns (μm) in diameter, some of which may deviate from the strands.

The second role is to provide compatibility with the molding process, the choice of which depends to a certain extent on the matrix resin to be used. The parent company Toray Industries, headquartered in Tokyo, Japan, is the world's major carbon fiber manufacturer and formulates its own rubber compounds. For example, Guillaume Deixonne, technical support engineer for industrial applications at Toray Carbon Fiber Europe (Paris, France), said that Toray has a specific size of carbon/vinyl ester suitable for marine environments. Like glass fiber, carbon fiber sizing can promote fiber impregnation and bonding between fiber and resin. This in turn improves the mechanical properties of the final product, such as compressive strength and interlaminar shear strength.

Although carbon fiber is used in more demanding applications, historically, carbon fiber sizing agents have not required as many formulators as they are designed for glass fiber. The latter sizing formulations are generally more complex than carbon fiber sizing formulations because they require a silane component to establish the bond between the glass and the polymer. This is not a problem with carbon fiber.

"For many years, the fact that carbon fiber itself brings very high mechanical properties to parts has been considered good enough, especially in an epoxy matrix," commented Michelman (Cincinnati, Ohio, USA) LeMoigne. Epoxy resin itself is "sticky", so it can wet organic carbon fibers well, but not all thermosets and thermoplastics are like this. In the future, carbon/epoxy composites are unlikely to dominate the market as they are today. "In order to achieve the next generation of composite materials that require higher performance, the interface between carbon and resin must be as effective as possible." It turns out that with the development of thermoplastic matrices, this is particularly challenging because they are inherently high.的viscosity.

"There are many different types of thermoplastics," Deixonne said. "We can't develop a one-size-fits-all formula." Riley and Deixonne agreed that the challenge is to develop a sizing solution that works well at high temperatures while ensuring that it provides a sizing solution that is compatible with unidirectional tow or fabrics produced by intermediate processors.

One of today's biggest challenges is to manufacture sizing chemicals and carbon fiber to meet the pressure of auto parts manufacturing during its production cycle.

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