1. Field of the Invention
The present invention relates to a process for producing formed structures using specific forming solutions comprising aromatic polyamides.
2. Description of the Related Art
Aromatic polyamides--also known as aramids--are known fiber-forming materials of high chemical resistance. Aramid fibers are notable in particular for good mechanical properties, such as high strengths and moduli.
Aramids are usually produced by solution polycondensation of dichlorides of aromatic dicarboxylic acids with aromatic diamines and either formed directly from the solution or precipitated from the solution and converted by renewed dissolving in a suitable solvent into a forming solution. The polycondensation gives rise to hydrogen chloride, which is unwelcome in the forming stage and is usually bound by neutralizing the reaction solution with a base. Usually the forming solution has salts for enhancing the solubility of the aramid added to it or such salts are formed as a consequence of the neutralization of the reaction solution.
It has also already been attempted to produce formed aromatic polyamide structures from salt-free forming solutions.
For instance, DE-B-2,204,075 describes forming solutions which consist of an aromatic polyamide consisting mainly of meta units and a specific N-alkyllactam as solvent.
Furthermore, DE-B-2,225,735 describes a process for preparing homogeneous forming solutions wherein an aromatic polyamide consisting of mainly meta units is suspended in N-methylpyrrolidone (NMP) in the absence of salts within a certain temperature range and dissolved in the NMA by heating to a certain temperature range. EP-A-522,418 discloses a process for producing solvent-spun aramid fibers. The processes for which concrete descriptions are given involve the use of forming solutions which are obtained by polycondensation of monomers to give the aramid and subsequent addition of neutralizing agent. These forming solutions are then directly spun.
There continues to be a need for processes for producing formed structures wherein forming solutions can be used in forming processes directly and without further process measures following the polycondensation of the aramid.
It has now been found that certain para-aramids can be polycondensed in specific solvents and directly converted into formed structures without a neutralization step. This omission of the neutralization step means an appreciable facilitation of process management, since a process stage can be dispensed with. It was further found that unneutralized solutions of certain para-aramids in certain solvents are stable over certain temperature ranges and that these solutions have significantly better filtering properties than corresponding neutralized solutions.