Intangible Assets

The gross carrying amount and accumulated amortization for intangible assets consisted of the following ($ in millions):



	September 30, 2017
	Gross Carrying Amount		Accumulated Amortization		Accumulated Impairments		Accumulated Foreign Exchange			Net Carrying Amount
Amortized Intangible Assets
Advanced Materials & Structures
Customer relationships	$	276.3	$	28.5	$	—	$	(1.2	)	$	249.0
Developed technology	34.0		5.4		—		—			28.6
Customer contracts	0.9		0.9		—		—			—
Trade names	21.8		0.8		20.7		0.3			—
Non-compete agreements	2.9		1.0		—		—			1.9
Patents	1.5		0.2		—		—			1.3
Total	$	337.4	$	36.8	$	20.7	$	(0.9	)	$	280.8

Unamortized Intangible Assets (Advanced Materials & Structures)
Trade names	$	20.0	$	—	$	—	$	(0.3	)	$	20.3



	December 31, 2016
	Gross Carrying Amount		Accumulated Amortization		Accumulated Impairments		Accumulated Foreign Exchange			Net Carrying Amount
Amortized Intangible Assets
Advanced Materials & Structures
Customer relationships	$	168.3	$	15.9	$	—	$	3.1		$	149.3
Developed technology	15.9		3.6		—		0.4			11.9
Customer contracts	0.9		0.9		—		—			—
Trade names	21.8		—		20.7		0.3			0.8
Non-compete agreements	1.7		0.5		—		—			1.2
Patents	1.5		0.2		—		—			1.3
Engineered Papers
Customer relationships	10.0		10.0		—		—			—
Total	$	220.1	$	31.1	$	20.7	$	3.8		$	164.5

Unamortized Intangible Assets (Advanced Materials & Structures)
Trade names	$	12.9	$	—	$	—	$	(0.1	)	$	13.0

Amortization expense of intangible assets was $5.2 million and $3.0 million for the three months ended September 30, 2017 and 2016, respectively, and $15.7 million and $9.2 million for the nine months ended September 30, 2017 and 2016, respectively. Incremental amortization expense of intangible assets for the Conwed acquisition was $2.2 million for the three months ended September 30, 2017 and $6.0 million for the nine months ended September 30, 2017. Finite-lived intangibles in the AMS segment are expensed using the straight-line amortization method. The estimated average aggregate amortization expense is $20.4 million in each of the next five years.

In December 2016, the Company made a strategic decision to transition away from certain legacy business trade names associated with its recent acquisitions in the AMS segment in favor of a streamlined SWM branding approach. As a result of adopting this branding strategy, in the fourth quarter of 2016, the Company recognized an impairment expense of $20.7 million, representing a write-down of the DelStar trade name intangible asset to its fair market value. The remaining value of this trade name was amortized over the first six months of 2017, as the DelStar trade name was phased out.