Elastomeric fibre

Elastomeric fibre 

What are Elastomeric Polyurethane Fibres ?

According to ASTM definition: ‘An elastomeric material is one which at room temperature can be stretched repeatedly to at least twice its original length and upon immediate release of stretch, will return with force to its approximate original length.’

The elastomeric fibres are required for making clothings conform to, extend with and physically support the human body. To meet the requirement of textile industry these are required as threads of 50-100 micron diameter, extensibility of at least 400%, easy stretch for comfort, rapid, forceful and complete recovery, high enough tensile strength for machinability, good whiteness index and dyeability.

These properties can be achieved if the material is capable of existing in two states:

Relaxed/coiled state Stretched or aligned state

The coiled state must be above its glass transition point at room temperature. The polymer should have high internal mobility to give low modulus and rapid retraction. For rapid retraction, there must be some degree of long range intermolecular interactions.

Vulcanized natural rubber meets these requirements. However, it has low strength, high recovery force, poor oxidative stability and dyeability. Hence there was a need to develop synthetic polymers fulfilling the above requirements.

Segmented polyurethanes have been reported to be suitable in this category and some are suitable for textile applications. Before we understand the segmented or elastomeric polyurethanes, let us see what are polyurethanes ?

The term polyurethane is used to describe polymeric materials with predominant urethane linkage

[-NH-CO-O-]

The fibres that meet the molecular requirements for rubberlike threads and are based on polyurethanes are categorized as elastomeric polyurethane fibres. Interestingly, the first polyurethane fibre spun in Germany was not an elastic yarn but was an ordinary “hard” yarn, rather like nylon. It was made by reacting 1,4 butanediol with hexamethylene diisocyanate, the two adding together to give the polyurethane. This polymer was entirely a polyurethane unlike the current elastomeric fibre.

nButane diol + hexamethylene di-isocyanate = polyurethane

Elastomeric polyurethanes are segmented polyurethanes and the word "polyurethane" describes polymers with significant number of urethane groups, together with a variety of other structurally important functional groups, such as ester, ether and urea groups.

The segmented polyurethane is a block copolymer of a soft and hard segment. The soft segments are very much longer and flexible than the hard segments, exhibit very low intermolecular forces, and provide the low modulus and high extensibility to fibre. On the other hand, the hard segments exhibit very high intermolecular forces which derive from rigidity, high degree of hydrogen bonding and crystallinity.

Polymerization Chemistry

In the polymerization process, a high molecular weight diol (polyol) is reacted with two moles of diisocyanate to form a prepolymer having isocyanate groups on both ends. The function of the diisocyanate is to convert the two hydroxyl end groups in polyol to diisocyanate ends. But inevitably some diisocyanates link glycols and therefore dimers, trimers etc. are formed. This chain growth can be increased by dropping the diisocyanate ratio below 2. The capped polyol or macrodiisocyanate is oligomeric with a characteristic molecular weight distribution. It contains unreacted diisocyanate, because for every diisocyanate that links two glycols, another is left unreacted.

The high molecular weight diol or polyol can be either ether based or ester based depending upon the type of linking groups. Polyoxytetramethylene glycol (PTMG) is an example of ether based polyol, while polyadipate and polycaprolactone are ester based polyols. Diphenyl methane 4,4’-diisocyanate(MDI) or toluene-2,4-diisocyanate(TDI) is used as capping agent.

Figure 1. Polyurethane Linkages

Chain Extension

This prepolymer is then reacted with a chain extender i.e. a diamine or a diol to form a high molecular weight polyurethane. On reaction with isocyanate group, the diamine chain extender molecule will form polyurea rigid segments and diol chain extender molecule will form polyurethane rigid segments. This fundamental difference between the diol and diamine extended materials leads to differences in physical properties between the two classes.

Most commonly used diamine chain extenders are hydrazine and ethylene diamine. The schemes below show the chain extension by formation of urea and urethane linkages.

(a) (b) Figure 2. Chain extension reaction

As mentioned above, there is a large amount of free diisocyanate in the prepolymer. This free diisocyanate reacts with chain extenders and forms the hard segment of polyurethane. The hard segments blocks grow between the preformed soft segments. The soft-segment domains are random-coiled aliphatic polyethers or co-polyesters. Because of this, the final polymer has a segmented structure comprising of both soft and hard segments. The structure of the final segmented polyurethane can be best explained by the following model.

One example of elastomeric fibre is Spandex fibre. Lycra or spandex was developed in 1959 by Dupont. It is a copolymer of high molecular weight poly tetra methylene glycol or caprolactone or polyethylene adipate as soft segment and diphenylmethane-4,4’-diisocyanate or toluene-2,4-diisocyanate as hard segment.

Chemical Structure of diisocyanates:

Chemical structure of some polyols: In general the soft segment has a very low melting point,if any,and is based on a polyether or polyester of a molecular weight of the order 1000-3000.

(-OCH2CH2CH2CH2CH2-CO) Polycaprolactone

Chemical structure of some chain extension agents:

H2NCH2CH2NH2 Ethylenediamine H2NNH2 Hydrazine

Fibre Formation :

The elastomeric fibres can be spun both by solution (both dry and wet) as well as melt spinning technique. The nature and amount of comonomers can be selected depending upon the spinning technique to be employed. The dry spinning is commercially the most popularly exploited method for spinning of Spandex. The wet spinning can be carried out either by spinning of final polyurethane solution or by spinning of prepolymer.

In the normal wet spinning, the solution is extruded into a coagulation bath through a spinneret and after coagulation, combination and fusion between the filaments take place, the fibre is wound up. The spinning of prepolymer is known as reaction spinning.

Figure 3. Different routes for polyurethane spinning

Dry spinning

The majority of elastomeric fibres made by solution spinning are dry-spun products having a polether –urethane structure. The dry spinning dope, containing at least 25% polymer, is metered through a filter pack and spinnerets at the top of a vertical spinning tube at required temperature. In the presence of heated inert gas flowing through the cell, nearly complete removal of solvent occurs as the filaments descend through the cell and becomes thinner. Unlike most synthetic fibres, these fibres are not subjected to an extensional drawing stage but are used in as-spun form. After emerging from the tube, the fibre is twisted. The twist travels upstream along the fibre to a point in the upper part of the tube where the filaments comprising the fibre fuse together and form an aggregate of filaments. This fusion is indispensible for subsequent processing of the fibre to prevent damage. The twist imparted is removed by the time the fibre reaches the first roller by means of the tension of the fibre itself. Then the finish oil that includes anti-tack agents is applied by contact with the finishing roller to prevent yarn adhesion on the package and is wound up by way of second roller. As shown in the Figure 4, the heated gas is supplied perpendicular to the fibre as in melt spinning. This prevents gas turbulence and heat transfer fluctuations. Therefore accelerates the solvent evaporation and decreases the entanglement of fibres and denier fluctuations.

Figure 4. Dry spinning of polyurethane

For increasing the spinning speeds, the main controlling parameter is the solvent evaporation rate. Therefore, for increasing the spinning speed would require:

Methods to supply heat to evaporate solvent. Technology to diminish tension High speed twisting technology High speed winding technology( low modulus so difficult to wind)

Reaction Spinning:

In reaction spinning, the macro diisocyanate or the prepolymer is extruded in a bath containing chain extender. The chemical reaction (chain extension) and the final filament formation occur simultaneously hence the name “reaction spinning”.

The final filament formation and chemical reaction in the filament occur almost simultaneously. This is the second most important spinning process employed for spandex after dry spinning. The technique exploits the high reactivity of diamines with diisocyanates. Prepolymer formed from either polyether or polyester macroglycol and diisocyanate, mixed with pigments and stabilizers is metered through spinnerets in the coagulation bath containing diamine. As the diamine diffuses to the surface of extrudate, chain extension reaction occurs rapidly to form a gradually thickening skin of block copolymer containing urea hard segments. The fibres made in this way are incompletely reacted on leaving the diamine bath and therefore bond with one another within the multi-filament yarn structure. The product is therefore fused multi-filament yarn, in contrast to the solution spun products.

The filaments are guided out of the bath before the reaction is complete (they still contain a core of fluid prepolymer).It is supported on a belt and cured in an oven to remove the volatiles and complete crosslinking reaction. Reaction spun spandex filaments are collected in bundles at the bath exit as in wet spinning. This process offers economic advantages in polymerization and solvent recovery. However, it has a product limitation mainly because of the bath drag on the filaments while they are soft and not fully converted. Therefore, the spinning of lighter fibres by this technique is difficult. Another limitation of this process is difficulty in controlling the final structure and molecular weight distribution.

Melt Spinning

Melt spinning of Spandex polymers is carried out on a small scale. In order to avoid degradation during melt spinning process, the polymer with polyurethane linkages rather than polyurea hard segments are preferred. The milder intermolecular attractive forces in polyurethane segments result in reducing the final spinning temperature. However, melt spun fibres can be produced from both polyether and polyester macroglycols reacted with diisocyanate and chain extender ethylene glycol or 1,4 butane diol.

For melt-spinnable polymers, chain extension is carried out as described above, with two exceptions. The solvent is omitted, of course, and the chain extender is a low molecular weight diol with primary hydroxyl groups (e.g, ethylene glycol). A basic or metalloorganic catalyst of the type described earlier is useful for accelerating the reaction. Since diols are principal ingredients of both the hard and soft segments, melt-spinnable polyurethanes may be produced alternatively in a single-step reaction of diisocyanate with a mixture of macroglycol and chain extender. Because of the statistical nature of the reaction, however, one-step synthesis gives a more polydisperse block copolymer with less well defined hard and soft-segment domains and poorer fibre properties.

The elastomeric fibres have elongation at break of over 400% with good tenacity and completely elastic recovery, as shown in Figure 5. This is in contrast to other type of stretch yarns made by ‘texturing’ or crimping a thermoplastic fibre,such as polyester or nylon.

Figure 5. Elastane stress/strain curve for elastomeric fibres

Structure-Property Correlation

The flexible soft segments comprise the predominant and continuous phase of the fibre, usually 65-90% by weight. In the relaxed fibre, they are essentially unoriented but uncoil and straighten to form crystalline regions during alignment. This results in stiffening and strengthening of the extended fibre. On release the soft-segment crystallites melt and the chains recoil with a force derived largely from entropy change. The length and nature of the soft segments determines the maximum extensibility of fibres.

Aromatic diisocyanates condense rapidly with the other glycols and diamines without any elimination product. Therefore is a preferred choice in hard-segment formation. The hard segments are commonly aromatic-aliphatic polyureas. The hydrogen bonded interactions between urethane groups and urea groups contribute to hard segment domain formation. The hard segment blocks are connected to soft segments by urethane linkages.

During fibre formation, the hard segments from several chains get associated into strongly bonded clusters or "tie-point" domains. They comprise less than 25% of the mass and form islands of a discontinuous phase and convert the polymer to a three dimensional network. The main forces of attraction are hydrogen bonds between NH groups and carbonyls. As mentioned above soft segments form a major fraction and this results in a easy (that is low modulus) and high degree of stretch.

Hard segments affect physical properties such as modulus, hardness, tensile strength and performance at high temperature. Higher conc. of polar groups in hard segments, cause an increase in cohesive forces and hence superior physical properties. On the other hand, soft segments determine or control the elongation or stretch in these fibres. Polyether based soft segments exhibit higher extension and lower physical properties than polyester based soft segments (due to weaker chain interactions). Polyethylene adipate gives high tensile strength as it crystallizes on extension. Increase in molecular weight of soft segment causes a decrease in modulus and increase in elongation at break. Generally soft segment -60-90% by weight and hard segment <25 div="">

Figure 6. Schematic representing hard and soft segments of elastomer

The hard and soft segments are spatially separated and form discrete domains. Polyester based elastomers show a lower level of crystalline order compared to polyether based polyurethanes. The lower crystalline order in polyester based elastomers may be attributed to the more irregular chemical structure. On extension, soft segment polymer chains undergo stretching and disentanglement, causing rigid domains to lie in a disoriented manner. In the case of polyether based elastomers, extension ~150% results in marked elongational crystallization of soft segments.

As the elastomer is further elongated upto 500% extension, the orientation of soft segments improves only to a small extent, while the hard segments get oriented in the direction of elongation as the maximum loading of chains in the soft segments oppose any further extension. Therefore, further extension causes a sliding process between hard segments.

At ~ 500 % extension, this process is complete and treatment of extended sample with warm water (80 °C) results in uniform distribution of forces amongst soft segment chains. This causes a loss of elongational crystallization. On relaxation the soft segments disorient completely, while the hard segments tend to remain in oriented form.

The various mechanisms of hydrogen bond disruption or physical relaxation in hard segments are shown schematically Figure 7.

Figure 7. Mechanism of hard bond disruption & relaxation in hard segments