Matrix viscosity and working time are the two major differences between VARTM and RTM. For RTM, the matrix must travel through the "X" and "Y" direction of a compact reinforcement, which requires using less then 200 cps for several hours. VARTM travels on top with transport media and only needs to impregnate the "T" or "Z" direction. This requires shorter time and can take advantage of the lower temp and faster vitrification/cure time combined with reduced thermal stress.

Because RTM requires lower viscosity, higher injection temperature (typically 160ºF - 250ºF range) is required for high-end matrix resins. The slow reaction rate at injection temperature translates into long latency at ambient and below, creating a single component system. For smaller parts, faster, two component RTM resins should be used and would overlap with the slowest of VARTM matrix resins.

Epoxy based RTM and VARTM matrix technology is dominated by the converters and modifiers. All vendors use similar epoxy resins. Converters can be divided into three categories and the amine class further divided. Processing, cost, kinetics, viscosity and end composite requirements dictate the converter class.

The three classes: aromatic, cycloaliphatic, and aliphatic, result in different cure rates and end properties but cure mechanism is the same. One amine hydrogen is available for one epoxy, which is known as stoichiometric i.e.: 1:1. This mechanism is very process forgiving as there are essentially no side reactions. (without catalysts)

The anhydride functionality does not react with an epoxy ring and must first be broken with another functional group, usually a hydroxyl or water. In contrast to amines, anhydride reactions are multiple. Water reacts directly with anhydrides to form a di acid, which lowers Tg, and slows time and temp to cure. After anhydride ring is opened, all reactions are very slow and there is no equilibrium end point, i.e.: different time and temps will give different properties, (the significance of which is T.B.D.)

Cyanates are well known for their database in space and low dielectrics for radomes, but not as well known as a converter for epoxies. Cyanates will react directly with the epoxy ring to form a 5 membered oxazolidone ring along with some cyanate/cyanate to form a triazine. This epoxy/cyanate reaction to an oxazolidone is the only epoxy cure mechanism where a heterocyclic ring is formed and a residual hydroxyl is not formed which translates into greater wet strength retention and higher wet and dry mods, especially at elevated temperatures. This mechanism is most probable for achieving high-end hot wet pre-preg type properties with Vartm processing, but toughness is an issue.

Small amounts of catalyst, either basic like a tertiary amine, Dicy, or imidazole family, or acidic such as a or a BF3 or BCL3 complex can be added to any epoxy to initiate or catalyze the epoxy functionality to react with itself i.e.: homopolymerize. Excellent combination of latency and cure rate can be achieved, but high temp wet and dry mods and fracture toughness are a problem

Modifiers are added for viscosity reduction and greater toughness, with as little reduction in mechanicals as possible. The low viscosity requirements eliminate most modifiers used in pre-pregs, which include thermoplastic oligomers and pre-precipitated particles.

With modifier addition, the resulting morphology is critical for mechanical/toughness performance. Two phase, bi modal particle size, creates a synergistic toughening mechanism that results in enhanced fracture toughness for best toughness balance. Two converter types: anhydride and cyanates, are intrinsically difficult to develop desired bi-modal morphology.

Applied Poleramic has developed desired two-phase modifiers for all of the converter classes. With 3D stitching the second phase may not be advantageous, as modulus suffers linearly with volume of second phase.

The material engineer has choice of Matrix property balances.

User-friendly VARTM resins should be impregnated at 140ºF or below, with ambient the goal. The higher the required balance of mechanical and toughness properties, the less likely ambient impregnation can be achieved. 140ºF and below requires a two component system.

API supplies all converter choices, single and two-component, together with a complete balance of mechanical and toughness properties. In addition to the product line, custom matrix resins are developed to meet required specs such as BMS 8-256, 276, 8-79 and 8-168.

All require elevated cure impregnation and frozen storage.

Converter choices: Anhydrides, slow Aromatic Amines, Cyanates and Catalytic. The anhydride and cyanates are catalytically controlled and catalyst choice dictates cure rate and latency.

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