Synthetic fibres account for about half of all fibre usage, with applications in every field of fibre and textile technology. Although many classes of fibre based on synthetic polymers have been evaluated as potentially valuable commercial products, polyester dominates the market.
A new and very important development in polyesters has been its usage in "aesthetic touch" fabrics, which were named "Shingosan" by the original Japanese inventors of this category of yarns. Most of the processes used were slow-speed technologies in which bi-shrinkage filament yarns are mechanical combinations of 2 or more different boiling-water-shrinkage property yarns. The mechanical combination is made with air-induced repeated continuous intermingled mixtures, twisted mixtures or cabled mixtures along the length of the yarns.
Bi-shrinking is a method to provide fabrics with texture in-situ during processing instead of the older method where yarn is textured first, with the yarn subsequently made into fabric. Due to differential shrinkage, the high-shrink component shrinks and the low-shrink component comes out of the fabric as micro-loops, giving the fabric excellent texture and handle. This provides the product with a very nice "textured" effect, "peach skin" effect, "brushed" effect or "terry loop" effect, depending on the fabric construction, and the component yarn characteristics, used.
Bi-shrinkage yarns - description
The yarns are a combination of two yarns of different shrinkages combined to create one composite yarn. The yarns are characterised by the special "SHRINKBULK EFFECT", ie when the fabric (knitted or woven) manufactured from them is subjected to wet-finishing (generally rotary-washer at 100 °C for woven cloth or pre-washer at 900 C for knitted cloth) or overfeeding at Stentor of the fabrics (140-165°C/6 chamber normal speed for knitted fabric), the yarn shrinks.
But the shrinkage differs from filament to filament. Thus, low-shrink components loop out from the core unlike the high-shrink components. The process has the potential to create yarns that can also replace textured yarns in certain applications. Texturing feed yarns is currently carried out separately using false-twisting, air-jet texturing, stuffer-box crimping, etc. Then, these yarns are used to manufacture various fabrics.
The technique with bi-shrinkage yarns was established by combining differential shrinkage/coil property yarns during extrusion/feed yarn production and then developing the texture in-situ during the fabric finishing process. The original Shingosan yarns are targeted at specialised, low-volume applications only due to the extremely high costs of manufacturing them. The high costs are the result of very slow-speed processes used during production as well as the multiple stages of production required to make the yarns.
The new process now breaks this barrier to using and developing bi-shrinkage yarns. In principle, BSY (bi-shrinkage) yarns result in a soft-touch fabric with a very different feel. Japanese researchers claimed that "it provides a brushed feel without the brushing". The Shingosan BSY fabrics are mostly split into the following 3 categories:
1) Peach skin - soft touch.
2) Silk-like fabrics with silky scroop.
3) "Shantung" effect - or wave-effect fabrics.
All these fabrics use different types of the BSY yarn family. We have tried to depict some of the combinations available on the market or referred to in literature and draw on the following acronyms:
FOY - Fully-oriented yarn, 2-step process.
PTY - Polyester friction textured yarn, 2-step process.
HSO - High speed-oriented yarn, 1-step process.
HOY - Highly-oriented yarn, 1-step process.
LOY - Low-oriented yarn, 1-step process.
FDY - Fully-drawn yarn, 1-step process.
SEY - Self-elongation yarn, heated 2-step process.
Fabric type Yarn type used
1) Peach skin FOY + FOY
PTY + FOY
PTY + HSO
FOY + LOY
FOY + POY
FDY + POY
SEY + FOY
SEY + FOY
2) Silk-like FOY + FOY (modified polymer)
FOY + FOY
FOY + SDY
SEY + FOY
(Normal bright circular or bright trilobal)
3) Shantung FOY + FOY
effect FOY + HSO
POY + HSO (modified polymer)
FOY + POY
FDY + POY
Description of current BSY product processes
Various production techniques are currently used in the production of bi-shrinkage yarns. The following are more commonly used methods:
Combination on godet draw-twisting machine concepts or draw-winding machine concepts
In this case, a special double godet draw-twisting machine is used. Existing machines can also be modified by installing a guide attachment and then passing one component through the plate heater and the second component without heater. In some draw-twisting machines, this modification uses a comb guide or separator guide, both of which are feasible and effective. In this case, it is possible to manufacture FOY-PTY and FOY-FOY types. The FOYFOY type is also being produced for bright trilobal products. This is a very stable and efficient method for producing quality.
Combination on normal hot-pin draw winding machine concepts
In this case, special double draw zone draw winders are available in both godet and non-godet types (ie, heated pin-type). The heated pin-type cannot be used for trilobal/bright yarns as drawing and yarn running is extremely unstable. For normal SD varieties, the non-godet type double draw zone can be used. Since the technique produces friction-wound packages, the best machine for manufacturing the various FOY-FOY, FOY-HSY, FOY-HSO combination yarns is the godet type double draw zone draw winder.
Combination of texturing machine concepts
This technique is not very popular and existing machine concepts can be used to produce BSY yarns following slight modifications and the installation of extra guides. Furthermore, the PTY-HSO combination, and other PTY combination types can be successfully produced using this technique on suitably modified texturing machines of various types available on the market. Please note that some suitably-modified texturing machines are also used in the Far East as a draw winder for producing FOY+FOY and FOY+HSO combination yarns in exceptional circumstances.
The yarn drawing is normally carried out for the low-shrinkage component, and the high-shrinkage component FOY, polymer-modified FOY or HSO is directly intermingled without using any heater temperatures in this case.
Please note that, in the event that POY, SEY, LOY components are used in the BSY yarns, you must consider the following important points in production:
1) The yarns have a use-by date and should be used within 10 months of production, as in the case of POY yarns.
2) The griege fabric should be finished or pre-finished (prepared for dyeing and shrinking process) within 20-45 days of production. Thus, griege stock as such, should not be kept for more than 45 days. If the griege stock needs to be kept for longer than 45 days, pre-finishing should be performed before storing the griege fabrics for longer periods. Dyeing and final finishing can then be carried out later. Storing griege fabric should not be done in hot ambient conditions in excess of 32°C.
3) Low-temperature dyeing at 100°C can be carried out, but in this case the yarn has a two-tone effect. If solid dyeing is required, high-temperature dyeing at 13°C is required.
Dyeing behavior of current BSY products
Because BSY yarn is a mixed structure of different yarns, the dyeing properties can be influenced by the characteristics of each fibre as follows:
1) Polymer type of each component.
2) Heat history of each component.
3) Shrinkage of each component.
4) Relative shrinkage differential between each component.
5) Interlace intensity.
6) Interlace type and profile.
7) Dynamic mechanical properties of each yarn in usage.
8) Fineness (or dpf - denier per filament) of each component.
9) Total relative size (or denier) of each component.
If we investigated the dyeing behavior of polyester FOY + POY type BSY yarns, such as 135den/108fil/BSY and 135den/96fil/BSY, the yarns reveal a distinct dyeing behaviour. The 135den/96fil/BSY/IM dyes more solidly, despite having less "loft". The polyester BSY flat yarns have very different dyeing properties in comparison to FDY/SDY. Their dyeing behavior is similar to that of easy-dyeable polyester or rapid dyeable polyester.
BSY yarns reveal very rapid dye absorption around Tg (Glass Transition Temperature), which sometimes causes uneven dyeing. Therefore, leveling time is needed at Tg for better evenness. BSY yarns are influenced by shrinkage effects due to the fibre assembly. Although a 100% solid shade cannot be obtained with the current BSY yarns, the two-tone phenomenon is reduced to some extent by dyeing at 1300 C using low molecular weight dyes.
Due to very difficult and conflicting matching of the 9 factors as specified above, perfect solid dyeing is a difficult task in current BSY production processes. In combination dyeing of BSY yarns, it has been observed that the low-shrink component in BSY yarns is more evenly dyed in the following order 135/96/IM > 135/108/IM > 135/144/IM. The shrinkage is also related to interlaced conditions, if the components have themselves been differentially dyed.
Hence, in the case of BSY yarn, the component yarns are differentially dyeable. The post intermingling or mixing technology adopted and its intensity of mixing has considerable influence on colour differences observed between the components in single bath dyeing or solidity of colour observed after single bath dyeing of the fabrics. Using higher interlace frequency in yarns results in more even colour and less of a two-tone effect.
Wherever the sophisticated two-tone effect is required, especially in "FAILE" ladieswear woven fabrics or "TWO-TONE" knitwear, this can be achieved by using different polymers or by using the components with higher dpf difference. This also permits the creation of very even and novel two-tone effects.
Description of new BSY product process
Barmag - in development co-operation with RMIT - has now introduced a novel new method of producing bi-shrinkage yarns that are highly cost efficient and at very high speeds in excess of 4800 m/min.
The newly-developed process is technically and commercially viable and has now been validated for manufacturing bi-shrinkage yarns using a modified combined process in conjunction with Barmag's Evospeed technology. This process is new and has been researched at the Barmag's R&D Centre in Remscheid, Germany. This system permits the spinning and direct combination of POY and HOY, or any other special yarn combination, in a single step.
This reduces production costs very significantly, while preventing unnecessary waste. The process has the potential to reduce the current costs of production of these yarns by at least half. The method used is shown in Figure 2. The adjacent positions have different control and can extrude different titre, different multifilament cooling rate, and different drag air acceleration. The position "A", we have spinning of individually controlled titer, radial multi-filament cooling and passive drag air accelerating layout. On the adjacent spinning position "B", we have a separate and distinct filament titer, cooling and active drag air accelerating layout.
With the right combination of these layouts, we have the theoretical possibility of obtaining very large shrinkage value differences in the filaments of up to 60% [without polymer modification] between the spun yarns from positions A and B. Simultaneously, these 2 filament combinations are combined before winding using special staggered intermingling or other specialised combining methods. One such layout is shown in the 3D depiction in Figure 2.
In this layout, position A has a specially-developed version of radial quenching and position B has a specially-developed version of Barmag's patented Evospeed quenching system. The process has been developed wherein at certain high spinning speeds in excess of 4500 m/min the positions A and B develop and produce yarns of vastly differing boiling water shrinkages and thus result in a high speed BSY at speeds and quality consistency not possible before.
After research and trials, yarns with shrinkage differences of 30% [without polymer modification] have been produced in a stable manner under long term running conditions.
Several other combinations and layouts are also currently under evaluation at Barmag's R&D Centre and testing facilities in Remscheid. We have successfully created micro-filament combination systems for such yarns. Our research is now concentrating on further-refining the process with the objective of establishing the market's very first industrial scale units.
Figure 3 depicts the trial configurations used from another perspective.
We have also already conceived and created systems for spinning different modifications of the polymer combinations. Processes and special yarn types for the same can also be developed and are currently under evaluation at the testing facilities as well. In exceptional circumstances, a system for manufacturing an SEY and POY type combination a single step is possible. Interestingly, this process does not use heated godets and hence is energy efficient, using "as spun" modifications in the quench phase.
Although for certain further modifications and special types, heated godets can also be used if the process demands it.
Another advantage of this technology is that experiments have shown that the 2 yarns combined in this manner reveal very consistent and even dyeing behavior and surprisingly solid-dyeing behavior has been observed.
This is attributed to particularly consistent and equivalent treatments in the spinning of the 2 yarns, which are combined. It should be pointed out that a solid-dyeing behavior in earlier slow-speed and 2-step processes is more difficult to obtain.
The research is now also concentrating on making finer total deniers using these BSY yarns, which marketing-wise have recently become more important and attractive to yarn producers.
BSY component selection for filament yarn aesthetics in fabrics
It is predicted that filament yarns will dominate the apparel market as a result of production efficiency. However, considerable work needs to be carried out on developing the aesthetics of filament yarns.
Historically, filament yarns have been the subject of considerable development, some of which we have outlined in Figure 4. To this end, it is clear that combined or combinational technology may have both aesthetic appeal and functional appeal in fabrics. Taking this into account, the new system lends itself to further modifications for such future-oriented developments.
Filament aesthetics developments
The combinations can be created for several purposes, some of which are depicted below:
1 Minimise negative, emphasise positive fibre property eg: hygroscopic with strength, combine crease resistance polyester + polyester with wicking profile.
2 Reduce costs: eg: making expensive fibres go further combine normal polyester + super microfilament polyester.
3 Combinational color effect eg: two-tone effects combine normal polyester + modified polymer polyester, combine cationic polyester + normal polyester.
4 Increase product uniformity: eg: make more even fabric surface combine fine microfilament polyester + coarse dpf polyester.
5 Increase product functionality: Eg: make water-impermeable breathable fabric combine super high-shrink polyester + polyester microfilament.
The above only depicts a few possibilities offered by the new system for fast and creative manufacturing of combinations.
Benefits of the new one step BSY process
1 1 Step process.
2 Significant production cost savings as spinning & combination is done in a single step and high spinning machine productivity due to high speed combined with winding of up to 5000 m/min.
3 Possibility of several new combinations in spinning due to independent positional control inverters and other attachment possibilities.
4 Dyeing differential between 2 components easier to manage due to simultaneous spinning of similar polymers at similar speed for both components. Can be enhanced or reduced as per the settings used.
5 Small lot production of specialties is possible with no left-over wastages due to unequal consumption of the 2 components as in earlier 2-stage processes.
Reasons for polyester BSY potentially replacing other filament yarns
This is an important next logical question to discuss and the relevant points here are as follows:
The efficiency of front-end textile processes
The front-end process of texturing for providing the filament yarn fabrics with aesthetics is currently very expensive and involves extensive resources in labour and energy. Wherever this new process is used, yarns can produce aesthetic fabrics of equivalent character, with BSY yarns now more economically favourable to DTY yarns.
Pet and co-pet polymer is inexpensive
Despite high oil prices, polyester polymer still remains the most cost-effective polymer for usage. Combined melt-spinning of components is the lowest-cost process available
Melt-spinning and the combined melt-spinning are the most economic yarn combinational processes available at the highest speeds.
Versatility, aesthetics and a new combination of technologies can be incorporated Further polymer modification developments with positional installations can result in further versatility to the process.
Textile properties still most moldable and best fit for most applications
Polyester polymer is still the best fit and the most mouldable polymer for fibre property adjustments as per the application requirements.
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Courtesy: Suprit Pal Singh, B Tech, MBA; Dipl-Ing Markus Reichwein and Dr Rajiv Padhye.