The term polymerisation defines the process of macromolecules formation through repetition of basic units: it of course applies only to synthesis fibres. In general, polymerisation reactions are activated and controlled during the process by various parameters, as temperature, pressure, catalysers, reaction stabilizers.
The number of repetitive units is termed degree of polymerisation and is a parameter of great significance for fibre properties setting. As the length of the single molecules is not constant, but varies according to a statistical model, the degree of polymerisation or the correspondent molecular weight has to be considered as an average value.
Depending on the various fibre typologies, the degrees of polymerisation may range from some hundred units in the case of polymers obtained through condensation (PA, PES) to some thousand units in the case of polymers resulting from poly-addition (PAN, PP). Under a production and application point of view, the degree of polymerisation is controlled by measuring following parameters:
Relative viscosity ηrel= solution viscosity/solvent viscosity = flow time t1/flow time t2
Intrinsic viscosity ηintr= ηrel /c →0 (concentration vanishing)
Melt flow index MFI = speed rate of the melted polymer at pre-established conditions
Relative viscosity is a parameter which is mostly used to identify nylon, while intrinsic viscosity (obtainable from the relative viscosity also by means of formulas) is used for polyester and the melt flow index for polypropylene. There are basically two mechanisms of chemical reaction available for the synthesis of linear polymers:
Poly-condensation: with this operation two molecules of same type or of different types are joined together to form macromolecules by removing simple secondary products as water, hydrochloric acid, alcohol.
The prerequisite for reactions of this type is the presence in the molecule (monomer) of two terminal reactive groups with functional properties. The molecules composed of 2,3,4…n monomers are named dimers, trimers, tetramers (oligomers)…polymers.
Some of the mostly used monomers are:
Aliphatic di-acids HOOC-R-COOH (used for nylon 6.6)
Aliphatic di-amines NH2-R-NH2 (used for nylon 6.6)
Aliphatic amino acids H2N-R-COOH (used for nylon 6)
Aromatic di-acids HOOC-Ar-COOH (used for polyester)
Diols (bi-functional alcohols) HO-R-OH (used for polyester)
Thus formed polymeric chains contain, besides carbon atoms, also various atoms (etero-atoms) resulting from the condensation reaction of the functional groups (e.g. nitrogen for polyamides, oxygen for polyester).
b) Poly-addition: this operation joins together several molecules and redistributes the valence links existing in the monomers, however without removing secondary products.
Many unsaturated compounds which are characterized by the presence of a double link between two adjacent carbon atoms as ethylene and its derivatives, polymerise according to this reaction; within this category fall e.g. acrylic and polyolefin fibres.
Among the most used polymers there are ethylene derived molecules with one or more
substitutes of hydrogen atoms.
Where X=H,CH3,Cl,CN,OH and other groups.
The chains which are thus formed originate from simple openings of double ethylene links and are therefore characterized by links only among carbon atoms.
Difference between addition and condensation polymerisation processes
Through poly-addition not only secondary substances are removed: reactions follow a chain
process, are quicker, highly exothermic and usually require lower temperatures.
Molecular weights (degree of polymerisation) are higher and it is more likely to have chains
with cross or branched links.
Polymerisation, once it is completed, does not leave behind polymers of intermediate length
(oligomers), but only non-reacted products (monomers).
Poly-condensation, on the contrary, is a process in several stages which leaves behind, among
reaction products, also polymers with low molecular weight (oligomers).
From a processing point of view, the polymerisation can be carried out by mass treatment, solution or dispersion (suspension, emulsion). From the engineering point of view, the process can be:
• discontinuous, where reagents are entirely pre-loaded into the reactor and, as soon as the polymerisation is completed, the products are completely unloaded. The “batch” technique is used in particular for the production of small lots or of specialty items.
• continuous, where reagents are introduced from one end and reaction products come out from the other (this process is used especially for large productions). The reaction can also take place within a stationary phase (as typical for poly-additions) or at subsequent stages (as in poly-condensations).
Whichever polymerisation method is applied, the reaction products (polymers) can appear as follows:
• in form of a solution to be conveyed to the spinning department;
• in form of a melted polymer to be conveyed directly to the spinning department or to be transformed into grains (chips) for subsequent use ;
• in form of a suspension, from which the polymer is separated and conveyed to the spinning department;
Along with the chemical reactants (monomers and possible catalysts) during the polymerisation stage or anyway in a stage preceding spinning, other additives can be added in order to provide the fibre with certain properties: a product of particular importance is a white dulling agent (titanium dioxide in grains), which is added in small quantities in order to give the fibres a “dull” appearance, which distinguishes them from the untreated fibres which, owing to their brighter and “synthetic” appearance, are named “bright”. Under this point of view, the fibre is termed on the basis of the added quantity of titanium dioxide (dullness degree) as follows:
• bright fibre: a fibre without or with minimal quantities of titanium dioxide;
• semi-bright fibre: a slightly delustred fibre
• semi-dull fibre: usually terms delustred fibres with 0,25-0.5% titanium dioxide contents
• dull fibre: fibre with 0,5-1% titanium dioxide
• superdull fibre: fibre with 1-3% titanium dioxide