2.5.1.1 EFECTOS DEL CORTISOL

directly after the airlay, so that the two stages can be performed sequentially. Barring that, the unbonded knoppy web would have to be rolled up and transported to the bonding oven. This would enable it to potentially sag under its own weight, pulling it apart and introducing holes that could compromise its thermal properties.

Bonding the PLA bres turns many slipping contact points into non-slipping ones, creating a skeleton amongst which the wool bres are threaded and interlocked. Given the relatively high number of PLA bres in knoppy web (section 2.5.5), this bonded substructure is expected to be extensive. It is important that eective micro-scale blending (section 2.5) has been performed and the PLA bres well-dispersed, both to ensure the pervasiveness of the skeleton and to avoid the generation of large hard lumps of PLA in the nal product.

The two parameters available within the bonding process are the chosen bonding temperature, and the time spent in the oven. The melting point of the outer sheath of bicomponent PLA is about130◦C,

so it is necessary to raise the internal temperature of the unbonded knoppy web above this point. This necessitates a higher oven temperature and a longer in-oven time, particularly for bulky knoppy web due to the fact that the outer regions of the knoppy web will insulate the inner regions. At the same time, the temperature at the outer edge of the knoppy web cannot be raised too high, or the wool will potentially suer discolouration [57].

2.9 Strategy and issues

As was alluded to in section 2.2, the sponsor company, FibreTech New Zealand Ltd, leverages all of the above tools and techniques for its knoppy web production strategies. The particular strategy enacted depends on the recipe and production volume, but the process outlined in g. 2.1 is generally accurate.

The choice of card is an obvious control strategy, albeit one that is mostly decided by the particular bre blend being used. The particular card used for each blend, and its particular settings (section 2.3), are outside the scope of this thesis; they have been determined through the decades of prior product development conducted at the factory.

Pre-melling is used to help blend the PLA bre into the wool. The PLA bre is passed through the opener with a roughly equal weight of the wool component, and the opened bre is then carded. This results in a bre pre-blend with a uniform wool:PLA ratio of 50:50 by weight, although the number of PLA bres is signicantly higher (section 2.5.5). In a subsequent step, the pre-blend is opened with the remainder of the wool component (which has separately been opened and carded) and then carded to obtain a nal sliver with a wool:PLA ratio of the desired target. Taking a target wool:PLA ratio of 85:15 as an example, pre-melling turns a single 85:15 blending step into a 50:50 blending step followed by a 70:30 blending step.

The major sticking point with blending is achieving the desired target ratios. The process used for blending the PLA bre into the wool bre stock uses the opener and card three times each, with the primary ratio blending point occurring at the opener. Similarly, the process for blending knops with the web carrier bre is to weigh out the desired ratio and pass the two components through the opener. There are several issues with this process:

• This is not a particularly ecient process; the time it takes to prepare the correct blend of compo-

nents in the small feed hopper of the opener (section 2.5.2) is non-negligible.

• It assumes that the ne PLA bres will blend more eectively into a smaller quantity of wool bres.

More specically, if the pre-melling is done with an initial wool:PLA ratio of 50:50 by weight, there will be a larger number of PLA bres present (section 2.5.5).

• Opening and carding all the bre twice increases processing losses due to bre breakage. This adds

to the overall cost of the operation.

• Additionally, there is no guarantee that the sliver emerging from the rst pre-melling step is actually

at the target ratio. Not that there is any guarantee of this for any carding step, but the two- step measuring process increases the likelihood that the resulting web blend is not at the target specication.

Nevertheless, with the opener being the entry point to FibreTech New Zealand Ltd's production line, it is the only place where component ratios can really be set. Alternative blending tools (if there were any that didn't have these issues) would incur a large capital cost that is not economic given FibreTech New Zealand Ltd's scale of operations.

There is also the existing knowledge base to consider. All of FibreTech New Zealand Ltd's products, and the knowledge and IP built around them, have been developed using the opener as the initial component blending point. Even if the nal product's component ratios do not match the recipe, its properties have been associated with the expected ratios. It is not outside the realm of possibility that altering the initial blending strategy could change the properties of the end product in unexpected ways. Ideally it would be for the better (by, for example, resulting in more even mixing of wool and PLA bres), but that is not guaranteed.

Shortening the bre stock is essential for knoppy web production, given that long bres have been seen during prior product development to be detrimental to knop formation. It is a particularly important con- trol strategy when the knops are desired to be small, due to the dependence on bre length (section 2.6). Cutting is therefore utilised due to the ease of control over the cutting length (by simply adjusting the speed of the sliver feed relative to the cutting blade frequency), and the tendency towards a shorter bre distribution than stretch-breaking (section 2.4). The prevalence of short bres (section 2.4.1) is in some sense benecial, although for particularly short cutting lengths it can be necessary to intentionally select a longer PLA bre length, to ensure that there is still sucient bre length to have cohesive bonding (section 2.8).

Blending via cutting (section 2.5.4) is a useful strategy for controlling the bre blend, and is used in blends where a weight ratio can be represented by a ratio of slivers. Implicit in this method, however, is the assumption that the linear densities of sliverAand sliverB are equal. In practice this assumption is reasonable at FibreTech New Zealand Ltd, for two reasons:

• The slivers that are to be cut and combined usually have about 50% of their bre content in

common, due to pre-melling (section 2.5.3).

• If the linear densities do dier (due to one sliver containing a particularly bulky bre, or due to

dierences in carding parameters), this would result in there being excess sliver of either A orB. The collected cut bre would therefore initially have a blend ratio that is light onAorB, but later switched to being solely comprised of that sliver. However, when subsequently fed into the opener's feed hopper (via bales or airstream) this would manifest as a layer at the top or bottom of the silo of pureAor B, which would be blended into the remainder of the bres via the silo blending step (section 2.5.2).

Without an on-site bonding oven, there is little else that can be done but transport the unbonded knoppy web to an o-site bonding oven as carefully as is practical. However, the use of an in-line bonding oven is not without diculties. One of the control parameters for the bonding stage is the time spent inside the oven (section 2.8), which is controlled by the speed of the oven's conveyor. With an in-line bonding oven, this conveyor must run at the same speed as the airlay's exit conveyorwhich is a key control parameter for the knoppy web's weight (section 2.7.1). Further testing would be necessary on-site to ensure that the bonding process is adequate over the range of conveyor speeds employed on the airlay,

2.10. SUMMARY 39