Intended purposes arguments are alive and well! 
In this case Sulzer Mixpac Ag owns patent number 7,815, 384, which is directed towards a device for mixing and dispensing Multi-components paints. A representative independent claim is reproduced below.
1. A device for applying a coating, comprising
at least two cylindrical cartridges,
a static mixing nozzle in fluid communication with the cartridges,
a spray tip, in fluid communication with the nozzle,
a first flexible hose is disposed between and in fluid communication with the nozzle and the spray tip, and
a second hose, in fluid communication with the spray tip, for supplying atomization air to the spray tip.
In this case, during an inter partes review of at the USPTO, the appeals board affirmed the Examiner’s decision that the claims are not obvious in view of Fukuta (US Patent 4,745,011) Jacobsen (US Patent 6,241,125), because Jacobsen discloses pumps, check valves, stop valves, and escape valves that are essential to Fukuta’s “principle of operation.” (i.e. preventing backflow even when the propensity for backflow occurs repeatedly and at a high velocity).
Specifically, the board reasoned that Fukuta is directed to the very manner in which the backflow of the mixture is prevented, hence the modifications suggested by Plas-Pak would impact this functionality in a fundamental way as to change the manner in which the apparatus of Fukuta functions. This is because the combination of Fukata and Morris (US 3989,228) would require substantial reconstruction” and “affect the principle of operation” disclosed in Fukuta. Furthermore, the board found that Jacobsen disclosed a device that dispenses fluid materials into a surface crack to minimize leakage, but does not disclose dispensing of fluid by spraying or that a spray nozzle would attain this objective.
In light of the above findings, the Board rejected Plas-Pak’s argument that combining Jacobsen with Hunter (2002/0170982) would have been nothing more than predictable variation of prior art elements according to their established functions. The Board reasoned that modifying Jacobsen to include Hunter’s spray nozzle, as Plas-Pak suggested, would undermine “the specific function of dispensing fluid materials directly into surface cracks to minimize leakage” and “render Jacobsen unsuitable for [that] intended purpose.” The Board thus held that the claims of the ’384 patent would not have been obvious over Jacobsen in view of Hunter. Plas Pak appealed the board’s decision.
On appeal, the board agreed with Sulzer that the board’s definition of Fukuta’s “principle of operation” and the Board’s finding that combining Fukuta and Morris would fundamentally alter that principle of operation. Specifically, the board held that the manner in which the two-component mixing apparatus of Fukuta prevents backflow is unique in its implementation, by replacing the valves and pumps of Fukuta’s system with the cylindrical cartridges and mixing gun of Morris, which fail to achieve comparable backflow prevention, would fundamentally alter Fukuta’s principle of operation. Accordingly, the board stated that such a change in a reference’s principle of operation is unlikely to motivate person of ordinary skill to pursue a combination.
HOUSTON PATENT ATTORNEY TAKE-A-WAYS:
1. This is one of the few cases I have seen where arguing the principle of operation was a winner without the primary reference specifically stating that it cannot be modified/used in a specific way. Patent Attorneys can use this case for support of their arguments in the future when an examiner broadly combines several references.
2. To build a patent portfolio what I would have recommended Sulzer do, is first amend the claims to get around the cited references. Then, file a continuation application fighting the good fight with the examiner at a later point. The main reason to do this is so you have a narrower patent that is granted, where a case can proceed without reexamination, while the second continuation application may be reviewed via inter partes examination. This can also be helpful when trying to form licensing agreements or settlement agreements before actually going to trial.