Leverkusen, Germany -- Bayer MaterialScience will present a technology for heat-activated dispersion adhesives, based on a new polycarbodiimide crosslinker, Desmodur XP 2802, at the European Coatings Show being held from 19-21 March 2013, in Nuremberg, Germany.
The technology offers particular advantages for uses where web-like substrates have to be bonded and it is important to form high strength rapidly at the joint, says BMS.
"Roll-to-roll bonding processes in the packaging and textile industries can benefit from this technology most of all," said BMS adhesive raw materials expert Jörg Büchner, in a company statement.
Polyurethane adhesive dispersions usually are processed with isocyanate crosslinkers, BMS says. But formulations using polycarbodiimide crosslinker display two unique properties that could not be combined when using isocyanate crosslinkers, the company explains.
First, despite being reactive, the formulations have high storage stability and stay suitable for use for several months. If Dispercoll U 2824 XP, for example, is used as the polyurethane dispersion, a pot lives of six months is possible "We developed this dispersion specifically to be combined with the polycarbodiimide crosslinker," Büchner said.
The second advantage is that the crosslinking reaction takes place immediately after drying at room temperature and a high rate. This allows bonded parts to be processed further almost immediately, BMS explains.
Tests by BMS show that if the crosslinking components are added correctly, crosslinking with the polycarbodiimide results in very durable bonds, even if the adhesive dispersion has been in storage for several months.
The polymer in Dispercoll U 2824 XP and the polycarbodiimide crosslinker in Desmodur XP 2802 are both present in the adhesive dispersion as separate droplets. In the bonding process the adhesive layer is dried after the formulation is applied to a substrate. During drying, the polymer and crosslinker droplets flow into one another and form a uniform adhesive film.
BMS says this film formation triggers crosslinking. If the adhesive layer cools before bonding, the polymer crystallizes in just a few minutes, forming a non-sticky polymer film. To bond two films or other substrates with it, it must be activated within one hour, with activation achieved by briefly applying heat, which melts the crystalline segments in the polymer chain.
After the substrates are pressed together, BMS says, the adhesive quickly hardens at room temperature by means of both crystallisation of the polymer's crystalline segments and the crosslinking reaction between the polymer and the polycarbodiimide crosslinker.
