Liquid (meth)acrylic syrup for impregnating a fibrous substrate, method of impregnating a fibrous substrate, composite material obtained following polymerisation of the pre-impregnated substrate

A liquid (meth)acrylic syrup for impregnating a fibrous substrate, said fibrous substrate consisting of long glass fibers of refractive index n1, wherein said impregnation syrup comprises:

• a) a (meth)acrylic monomer,
• b) at least one component, of refractive index n3, such that n3≥n1, wherein said component b) is selected from the group consisting of
• styrene-maleic anhydride copolymer, present in a proportion of at least 10% by weight of the total liquid (meth)acrylic syrup, said styrene-maleic anhydride copolymer comprising between 25% and 50% of maleic anhydride monomer units, and
• styrene monomer, present in a proportion of at least 10% by weight of the total liquid (meth)acrylic syrup;
• and
• optionally c) a (meth)acrylic polymer of refractive index n2, wherein n3>n2, and wherein said (meth)acrylic polymer c) may or may not be present if component b) is a styrene-maleic anhydride copolymer, but said (meth)acrylic polymer c) must be present when component b) is styrene monomer,
• said (meth)acrylic syrup having a refractive index n4 which is about the refractive index of n1 and a dynamic viscosity of between 10 mPa·s and 10,000 mPa·s.