Adhesive composition having a gel content based on cross-linking via keto groups or aldehyde groups

A pressure-sensitive adhesive composition in the form of an aqueous polymer dispersion, the pressure sensitive adhesive composition comprising:

• (a) a tackifier;
• (b) a compound A comprising at least two functional groups capable of undergoing a crosslinking reaction with keto groups or with aldehyde groups;
• and
• (c) a pressure-sensitive adhesive polymer formed by emulsion polymerization from
• (i) at least 60% by weight, based on a sum of the monomers, of at least one soft (meth)acrylic ester monomer having, when polymerized as a homopolymer, a glass transition temperature of less than 0° C.,
• (ii) 0.1% to 10% by weight, based on the sum of the monomers, of at least one monomer having at least one acid group,
• (iii) at least 5% and less than 20% by weight, based on the sum of the monomers, of styrene,
• (iv) 0.3% to 5% by weight, based on the total amount of the monomers, of at least one ethylenically unsaturated compound K having at least one functional group selected from keto groups and aldehyde groups, and optionally
• (v) one or more further monomers distinct from (i) to (iv),
• with the exception of monomers having at least two non-conjugated polymerizable vinyl groups,
• wherein the polymerization is carried out in the presence of 0 to 1 parts by weight of a chain transfer agent per 100 parts by weight of the monomers,
• wherein after film formation on a substrate, the pressure-sensitive adhesive polymer has a gel content geltot of at least 80% by weight based on a polymer film produced from the pressure-sensitive adhesive polymer,
• wherein the gel content geltot is at least partially based on a crosslinking of the keto or aldehyde groups of a polymer with compound A and a gel content gelK of the pressure-sensitive adhesive polymer based on crosslinking of the keto or aldehyde groups of the polymer with compound A is at least 30% by weight,
• wherein the gel content in part is optionally based on covalent, irreversible crosslinking not effected by crosslinking of the keto or aldehyde groups of the polymer with compound A and a gel content gel0 of the pressure-sensitive adhesive polymer based on covalent, irreversible crosslinking not effected by crosslinking of the keto or aldehyde groups of the polymer with compound A is from 0% to not more than 50% by weight,
• wherein a glass transition temperature of the polymer before crosslinking of the keto or aldehyde groups of the polymer with compound A is below 10° C., and
• wherein after drying, the pressure-sensitive adhesive composition forms a film having a tack of not less than 10 N/25 mm measured as loop tack relative to a steel surface at 23° C., 50% relative humidity and at a tearoff speed of 300 mm/min.