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17th January 2018, Cincinnati, OH

A critical science

Michelman Industrial Manufacturing’s marketing director Steve Bassetti. © MichelmanFibre sizing has an extremely significant impact on the final properties of composite parts. Michelman Industrial Manufacturing’s marketing director Steve Bassetti, Gilles LeMoigne (fibre and composites industry manager) and Nagesh Potluri (group technology director) explain.

Inside Composites: Please can you tell our readers a little bit about Michelman and its activities?

Steve Bassetti: Michelman was founded in 1949 and is well known for its capabilities in the area of emulsification and polymer dispersion technologies. Early successes with polypropylene sizing applications led to its acquisition of Hydrosize Technologies in 2008.

Since adding the Hydrosize brand, Michelman has continued to innovate around our materials and process capabilities. We manufacture a versatile line of fibre sizing and surface treatments that are used by fibre producers and composite manufacturers to produce stronger, lighter and more durable composite parts.

Michelman’s materials are used in the manufacturing of glass basalt carbon and natural fibres, and fabrics made from these fibres. We have become the interface adhesion experts because of our technology expertise and a diverse product range that is optimized for a variety of thermoplastic, thermoset and technical textile fabric applications.

Michelman is committed to serving this industry and is embarking on a multi-year investment strategy that will add capabilities and expertise to further advance its sizing and composite development.

What is fibre sizing?

Gilles LeMoigne, Michelman Industrial Manufacturing’s fibre and composites industry manager. © MichelmanSB: Fibre sizing is a thin homogenous coating applied on the fibre surface during manufacturing. It protects the fibre during its production, but also during post processing operations such as handling, spreading, weaving, compounding and moulding. It also provides adhesion to the matrix polymer used to produce the composite or technical textile. Typically, the fibre producer creates the final sizing formulation which will include ingredients such as coupling agents, defoamers, film formers, lubricants and other functional additives.

Why is it necessary?

Gilles LeMoigne: Sizing optimizes the processability of carbon fibre. Although sizing is a small percentage of carbon fibre composites – normally a 0.5 to 5% of its weight – it performs several tasks including protecting the fibre during its manufacturing, as well as processing it into intermediary forms such as fabrics, braids, thermoset prepreg and thermoplastic organo sheets.

Our technology allows customers to tailor the surface chemistry of reinforcement fibres to the chemistry of the matrix resin, thereby optimizing the interfacial adhesion between the polymers and fibres.

What are the components of a fibre sizing formulation?

Nagesh Potluri: As mentioned previously, fibre sizing formulations are composed of water, coupling agents (specifically for glass and basalt fibre), film formers in dissolved, emulsified or dispersed form, and a range of additives or modifiers such as  surfactants, plasticizers, anti-static agents, antifoams and rheology modifiers. These ingredients are then mixed together and delivered to the fibre as the final sizing formulation.

Nagesh Potluri, Michelman Industrial Manufacturing’s group technology director. © MichelmanOn the fibre surface, the film former plays an important role in determining the properties of the final composite, such as improved adhesion between the fibre and the matrix; the mechanical properties as it facilitates effective load transfer between matrix and fibre, along with chemical or water resistance, and thermal stability. Hundreds of film former dispersions are commercially available, but only a small percentage of them are compatible with other sizing ingredients.

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

NP: The sizing’s compatibility with the resin matrix is key to improved composite properties.

Sizing ultimately affects the production and the processing of the fibre, and has a significant impact on the interface properties between the fibre surface and the composite matrix.

The right sizing must render the fibre and matrix compatible and this is accomplished in large part by controlling the surface chemistry of the sized fibre. In addition, through proper design, an enhanced interface helps in matching the elongation at break between the fibre and matrix.

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

Figure 1: Carbon fibre production process. © Michelman

GLM: Carbon fibre production is generally a six-part process and sizing is applied near the end (as shown in Figure 1).  Every fibre producer develops their own sizing know-how and applies it to the fibre to best suit the needs of the targeted applications. We understand the role sizing plays in the manufacturing of composites as well as fibre, and we work with all members of the value chain, to formulate sizing for particular resin characteristics, as well as for specific properties desired in composites.

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

GLM: Sizing stability is the ability of a formulation to stay in a homogeneous state from the moment it is mixed to the time it is delivered onto the fibre. This stability is one of the key characteristics that carbon fibre manufacturers take into consideration when selecting raw materials for their sizing formulations. There are 4 major effects of sizing variation on fibre production (as shown in Table 1).

Table 1: Effect of sizing variation on fibre production. © Michelman

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

SB: When it comes to the manufacturing and further processing of fibre, changes in sizing film formers can benefit both the fibre physical characteristics and finished composite properties (as shown in Table 2). Sizing provides benefits in carbon fibre manufacturing such as protection of the freshly formed fibre surface from abrasion, prevention of fibre breakage at contact points, and preservation of the chopped bundles of fibre.

GLM: There are multiple ways that sizing protects carbon fibre during handling and processing. Not only does it provide integrity by allowing the chopped bundles to stay together, it helps in chopping because it increases the chopper’s blade life while reducing fuzz generation. In the case of continuous fibre both the unwinding and weaving processes are supported because sizing keeps the strands of fibre together while providing lubricity and flexibility that prevents breakage and aids in fibre spreading.

NP: One cannot overestimate sizing’s contribution to ease of handling. Weavers have worked with fibre manufacturers for decades to develop sizing that, when fibres are passed through a liquid bath, form a film or coating on the fibre surface, which effectively “lubricates” the fibre to optimize production speed and facilitate the weaving process in ways that produce more cost-efficient, higher quality carbon fibre fabrics.

Table 2: Benefits of fibre sizing. © Michelman

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

SB: Sizing enhances a number of mechanical and chemical properties in composites due to the role it plays at the interface of the fibre and the matrix. Failure modes for composites can occur in the matrix, at the interface, or in the fibre. Each application and its requirements will dictate the potential failure mode and the role that sizing can play in mitigating that failure.  

Sizing’s benefits to composites include:

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

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

NP: The type of resin, type of fibre, fibre processing, and final composite application are the four important considerations in sizing selection.

Thermoset or thermoplastic resins need different chemistries to form an effective interface between the fibre surface and matrix. Whatever is compatible with one thermosetting or thermoplastic resin is unlikely to be compatible with another. In the case of thermoset resins, during the curing process, the interface has a chance to cross link with the film formers on the fibres. On the other hand, special chemistries need to be delivered through sizing to induce the cross linking between film formers and thermoplastic resins.

In addition to conventional chemistries, Michelman has unique polyurethane, polyamide and polyimide sizing chemistries to effectively cross link fibre and thermoplastic resins through the film former. Michelman has active development programs geared towards creating products for the high temperature processable engineered thermoplastic markets that are generally underserved from a sizing standpoint.

GLM: Different sizing will be needed for chopped versus continuous fibres. For example, chopped fibre needs to have extremely good bundle integrity or the fibre will be difficult to feed into the compounding machine. But, continuous fibre applications might use a sizing that enables greater filament spreading for easier wet-out.”

Requirements for the composite part’s final application is also important in sizing selection. Sizing chemistry plays a significant role in a composite’s mechanical properties including impact resistance, tensile strength, and fatigue resistance, as well as its material and chemical properties such as corrosion, hydrolysis, heat, and oil-resistance.

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

SB: Hydrosize Link is an enhanced reactive sizing that increases the chemical resistance of polyamide composites. It is designed for long-term performance – including exposure to aggressive fuels which allows automakers to cost effectively meet emissions regulations and increase fuel efficiency.

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

Technically, Hydrosize Link is a series of polyurethane film formers designed to offer enhanced reactivity with the polyamide matrix resin.

NP: These high performance sizing products are added during the fibre manufacturing process and are activated during compounding to increase the interfacial adhesion between the fibre and polymer matrix. This improved adhesion provides great resistance to the glycol fluids used in coolant systems and allows the automakers to produce cost-effective components that perform to the exact specifications required.

Figure 2: Hydrosize Link performance. © Michelman

www.michelman.com

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