The sizing is a mixture of various chemicals, usually (but not necessarily) diluted in water, that fiber and fabric producers coat (“size”) their fibres with. The sizing has many functions, and that explains why it often is a very complex blend of ingredients: typically one or several polymeric components, a coupling agent, a lubricant and a range of additives (surfactants, plasticizers, anti-static agents, adhesion promoters, anti-foams, rheology modifiers, etc). This mixture is typically applied to the fiber in a rather dilute, aqueous form – solids between 5 and 15%.
The functions of the sizing can be related to production of the fiber, processing of the fiber and the interfacial properties provided by the sizing between fiber surface and composite matrix.
The following will focus on this last issue.
Interfacial properties and contribution to composite properties
Key questions for the developer of a sizing to ask are: 1. Which resin needs to be reinforced by the fiber, 2. Which process will be used to make a composite part with that particular resin – reinforcement combination, 3. What requirements does the final composite part need to meet?
Since one of the functions of a sizing is to form at least a temporary – and often a permanent – interface between fiber surface and matrix (or resin solvent), mostly every matrix requires a different sizing chemistry: whatever is compatible with one thermosetting or thermoplastic resin is unlikely to be compatible with another!
A major challenge for the sizing developer is that some of the sizing chemicals that were selected to be able to produce and process the fiber, are detrimental to composite properties – ‘they harm the interfacial properties’. Very often lubricants and surfactants are guilty at this stage. A chemical supplier often sells a recipe, not just a pure chemical; be aware that ingredients of a polymeric dispersion for example may result in undesired effects (the polymeric nature can be right, however the surfactant, biocide, pH modifier can be very wrong). Chemical companies with personnel specialized in serving your industry will anticipate and avoid most of these negative effects whilst developing products for use in sizing.
One can segment processes in many ways; here are just a few examples:
Furthermore, whether the composite is made of a thermosetting or a thermoplastic resin has profound consequences for the sizing chemistry; i.e., some of the main thermosetting resins come as dissolved in styrene, and wetting followed by controlled but complete dissolution of the sizing polymer in styrene is required. In case the fiber is used in a thermoplastic resin, the molten resin has to wet, but not thermally degrade the sizing, and the individual fibres need to disperse into the hot melt as uniformly as possible and with as little mechanical force as possible (maintaining a certain fiber length is key for mechanical properties).
The sizing chemistry contributes to the mechanical properties (impact resistance, tensile strength, fatigue resistance…) as well as other material or chemical properties (corrosion-, hydrolysis-, heat-, oil-resistance) of the composite part. The final colour of the composite part, its surface aesthetics, whether or not odour will be emitted by the part when installed in a new car, … the list goes on.
An increasingly important aspect of the sizing is its regulatory and safety footprint. REACh, food and drinking water contact approvals, country inventories, safety while using the chemicals during fiber production and of the fiber during composite production, level and type of emissions generated, … all these things require good communication between chemicals supplier, fiber producer and fiber user, before, during and after the development of a sizing.
Published courtesy of Michelman
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