2.4.3 MODIFICACIONES DURANTE LA GESTACIÓN

f ρf (g cm

−3₎

lf (mm) Df (µm)

Southdown wool 1.314 55 (cut) 29.7

PLA51 1.210 - 1.430 [53] 51 21.6 - 19.9 (4 Denier) PLA32 1.210 - 1.430 [53] 32 21.6 - 19.9 (4 Denier)

Table 2.2: Number ratios for dierent bre type blends.

f f0 mf/mf0 ρ_f/ρ_f0 l_f/l_f0 D_f/D_f0 N_f/N_f0

5.667 (85:15) 2.565 (72:28)

Southdown wool PLA51 4.000 (80:20) 1.086 - 0.919 1.078 1.375 - 1.492 1.811 (64:36)

2.333 (70:30) 1.056 (51:49)

5.667 (85:15) 1.610 (62:38)

Southdown wool PLA32 4.000 (80:20) 1.086 - 0.919 1.719 1.375 - 1.492 1.136 (53:47)

2.333 (70:30) 0.663 (40:60)

reasonably well understood; it is possible to create dierent kinds of knops targeted at dierent products. There are two obvious controllable parameters:

• The bre blendthe types and ratios of the wool and PLA bres used have a signicant eect on

the softness and resilience of the knops.

• The bre length distributionone of the primary factors in the size of the knops. It also aects

their formation; with even a small fraction of long bres, the resulting knops are observed to form tails, and be less well-formed. This enables knops to be incorporated into e.g. yarns, but is less desirable when creating knoppy web for bedding ll because the knops are less discretised.

There are also several other key controllable parameters that cannot be discussed here (as they are the proprietary know-how of the company).

Knops are very dierent to nepsthe latter are entangled/felted brous clusters, while the former are spherical clusters of curled bres. Suce to say that while neps have been studied extensively (e.g. [54, 55]), there is no existing literature describing knops or the knop-forming process. A theoretical treatment of the knop formation process is outside the scope of this thesis, but the knops themselves will be modeled as part of the development of a knoppy web model in chapter 4.

2.7 Airlaying

The knoppy web itself is formed by taking the knop and web components, blending them together, and then feeding the blend into an airlay machine. The physics of blending was discussed in section 2.5, and diers little here; suce to say that the primary concern is mixing the knops and web eectively without destroying the structure of the knops.

There exist a variety of airlaying technologies used for diering purposes [56], however only those that enable production of high-bulk products are of any signicance to knoppy web production. Figure 2.18 shows the airlay that is currently used by FibreTech New Zealand Ltd to produce knoppy web. The general principle follows that of carding (section 2.3): the bre blend to be airlayed is condensed in a hopper, and then a high-speed licker-in expands the blend into an airstream. The aerated blend ows through an outlet and against a pair of perforated rollers (g. 2.19), allowing the air to escape and condensing the blend into a batt.

For pure brous blends, it is claimed that the DOA airlay provides an excellent random distribution of bres throughout the batt, due to the air-blowing system used [56]. However, in the case of knoppy web blends, the licker-in has a signicant eect on the structure of the end product. The licker-in is

Figure 2.18: A DOA airlay machine producing knoppy web.

2.7. AIRLAYING 35

situated above the airstream (g. 2.20). Opened bres are released from the licker-in by centrifugal force; the knops are signicantly denser than the web bre, and are ung further into the airstream. This means that the knop:web ratio increases across the thickness of the end product, with more knops at the bottom and more web at the top. This will aect the compressional properties of the product (because the stronger knops are mostly on one side), but also the handle and feelthe upper side of the knoppy web will have a softer feel and more uniform appearance.

This aberration can in fact be used to our advantage. By placing two sheets of knoppy web together with the knop-rich sides facing inwards, products can be created that have a softer handle (from the higher web bre content at the surface), but with the structural benet of the knops at its core. The thickness of the product would be increased, but this can be mitigated by laying the initial sheets at lower weights. Modern airlay machines can in fact be designed to perform this automatically, simply by having two licker-ins spinning in opposite directions, laying the bre blend from above and below.

2.7.1 Target vs. actual weights

One of the important end properties of a knoppy web is its weight, measured in grams per square metre. This is an important quantity in bedding and apparel products, as it aects many user-relevant properties such as warmth and comfort. Duvets are regularly marketed to users by their weight, usually with summer and winter weights. Thus it is important that knoppy web can be produced to a particular target weight, and the key control point for this is at the airlay.

In practice, however, target weights must be taken as a guideline only for the production line. It is not possible to reliably produce an exact weight specication, simply because of the complexity of airlay control. There are several dierent parameters that can be tuned on a DOA airlay:

• Hopper feeder speed. • Licker-in speed. • Airow speed.

• Condenser screen height. • Exit conveyor speed.

The exit conveyor speed has perhaps the simplest relationship: speeding up the conveyor lessens the density of the web. The condenser screen height puts a restriction on the potential thickness of the knoppy web. Beyond this, the relationships between the various parameters become increasingly non- trivial to specify precisely, particularly when factoring in the eect of any non-uniform blending of the knoppy web blend. The only reliable way to determine the actual weight produced is to run the airlay for a few metres and then measure the outputbut the process of starting and stopping the airlay can itself aect the output weight. Achieving the target weight therefore comes down primarily to operator skill and experience.