Spandex is the most familiar elastic fiber that everyone is most familiar with. It has far better elasticity and elongation than other fibers. However, spandex fiber is not “soft” and is even harder than polyester, and the elasticity of spandex comes from this. ”
1 Synthesis of spandex
The scientific name of spandex is polyurethane elastic fiber, which is the main chemical component of spandex. First, take excess glycol (can be a mixture of ethylene glycol and propylene glycol) and adipic acid Reaction to generate polyester or polyether with a molecular weight of 1000 to 5000 and terminal groups of hydroxyl groups; then the polyester or polyether reacts with aromatic diisocyanate to generate a prepolymer with isocyanate terminal groups; finally, the prepolymer is mixed with an aromatic diisocyanate. The chain extender of the bifunctional group of active hydrogen atoms reacts to form a block copolymer.
Multi-step polymerization finally makes spandex form a “soft segment” and The alternating block structure of “hard segments” is also the secret of spandex’s ultra-high elasticity.
2 The source of elasticity
Homopolymer polyurethane fiber is not only hard, but also does not have good elasticity. The good elasticity of spandex is firstly due to the soft segment and the hard segment. It is caused by the network structure composed of block copolymers, that is, it has a “segment” network structure.
“Soft segment” is not crystalline and has a molecular weight Larger low-melting point (melting point below 50°C) polyester or polyether long chain, which forms the amorphous segment of the fiber, has a molecular weight of 2000 to 4000, low Tg (-50 to -70°C), and is in a highly elastic state at room temperature. , the molecular chain is curled, and it is easy to deform under the action of stress, and the fiber is easily elongated. But only the softness of the “soft segment” is not enough.
Like a spring, only Pulling one side of the spring will not extend and lacks a force point. Instead, it needs to pull both sides of the spring outward at the same time to extend and contract. This is due to the fundamental “hard chain segment” of elasticity. “Hard chain” “Segment” is composed of aromatic diisocyanates and chain extenders that can form hydrogen bonds, easily generate crystal structures or can produce lateral cross-linking, and are highly symmetrical. The crystalline segments of the fiber are formed through hydrogen bonds in adjacent molecular chains. The length of the hard segment is short, the molecular weight is 500-700, the melting point and Tg are high, the softening and melting range is 230-260°C, and there is basically no deformation under stress, which prevents slippage between macromolecular chains and is the largest soft segment. Amplitude elongation and rebound provide the necessary knot conditions.
The hard segments in the molecular chain have strong forces and are arranged neatly with each other to form crystal regions; the parts of the molecular chain in the soft segments that are not affected by external forces are in a relaxed state (bent or curled) ). After being subjected to external force, when the fiber is elongated to 200%, some of the soft segment molecular chains are stretched and arranged neatly, and even crystallize; when the external force is removed, due to the weak force between the molecular chains, in the crystallized state Under the action of the hard segment nodes, the stretched molecular chain will slide freely and become relaxed, then shrink back to the state of minimum stress, showing high elasticity.
3 Delicate and sickly spandex
“Hard segment “The glass transition temperature is 230℃~260℃, which is much higher than the 81℃ of polyester fiber. With such a stable “hard segment” fixed, why is spandex fiber still a “delicate and easy to get sick” fiber in our printing and dyeing processing.
Spandex rarely appears in the form of bare silk. A small amount of spandex mixed into the fabric can significantly improve the elasticity. In the printing and dyeing process of elastic jeans, the washing and aging process is indispensable. The maliu process of hydrogen peroxide, potassium permanganate, and ozone relies on strong oxidation to destroy the chromogenic groups of the dye, thereby achieving anti-aging and discoloration.
These strong oxidants can easily destroy the -O-CO-NH-polyether connecting the “soft segment” and “hard segment” in the spandex fiber. Amide group, this group is more reactive and easy to break and hydrolyze. As a result, the “soft chain segment” and the “hard chain segment” are separated, and the spring structure of the stressed node cannot be formed, resulting in elastic damage to the spandex. Not only are strong oxidants easy to damage spandex, but they are also prone to accelerate the hydrolysis and rupture of the connecting group under alkaline and high temperature conditions. Therefore, during the printing and dyeing process of spandex products, more attention should be paid to factors such as temperature, pH value, redox agents, etc.
The warp white silk phenomenon of spandex fiber is also a pain point in printing and dyeing processing. Based on the structural characteristics of spandex, although it can be dyed with acid dyes, neutral dyes, acid mordant dyes and disperse dyes, disperse dye dyeing is still the best choice.
In the disperse dye dyeing process, although spandex has a microphase structure composed of soft and hard segments, which makes the disperse dye easy to dye, it is also easy to desorb. This is because the current disperse dye products are mainly suitable for polyester dyeing. The dye has a smaller molecular structure and fewer polar groups, so it is easier to diffuse into the spandex, but it is difficult to establish a strong bond with the soft segment molecules of the spandex. combined, so the equilibrium dye pick-up is low and in wetEasily desorbed under hot conditions. In many cases, this property of disperse dyes seriously affects the color fastness of spandex-containing textiles.
In the daily printing and dyeing process of spandex blended fabrics, due to the low spandex content [spandex is generally made into core-spun yarn (the core is spandex)], spandex is generally not dyed. If the spandex is exposed and disperse dyes are used to dye the spandex, the dyeing temperature should not be too high and should be lower than 120°C. The fabric tension should not be too high, otherwise the elastic loss of the spandex will increase. The dyeing temperature can be reduced by using carriers or dyeing accelerators. At present, spandex dyeing mainly suffers from poor wet treatment fastness and difficult to control process. It is necessary to develop and select special dyes, such as some acidic, metal complex and disperse dyes, as well as new dyeing auxiliaries and dyeing processes. </p
