Intangible Assets

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



	December 31, 2017
	Gross Carrying Amount		Accumulated Amortization		Accumulated Impairments		Accumulated Foreign Exchange			Net Carrying Amount
Amortized Intangible Assets
Advanced Materials & Structures
Customer relationships	$	276.3	$	32.9	$	—	$	(2.3	)	$	245.7
Developed technology	34.0		6.0		—		(0.1		)	28.1
Customer contracts	0.9		0.9		—		—			—
Trade names	21.8		0.8		20.7		0.3			—
Non-compete agreements	2.9		1.1		—		—			1.8
Patents	1.5		0.3		—		—			1.2
Total	$	337.4	$	42.0	$	20.7	$	(2.1	)	$	276.8

Unamortized Intangible Assets (Advanced Materials & Structures)
Trade names	$	20.0	$	—	$	—	$	(0.4	)	$	20.4



	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-related intangibles	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 $20.9 million, $12.3 million and $5.6 million for the years ended December 31, 2017, 2016 and 2015, respectively. Incremental amortization expense of intangible assets for the Conwed acquisition was $8.2 million for the year ended December 31, 2017. Finite-lived intangibles in the AMS segment are expensed using the straight-line amortization method.

In our AMS segment, the Company made a strategic decision to transition away from certain legacy business trade names associated with its recent acquisitions 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, leaving a remaining balance of $0.8 million, which was fully amortized over the first six months of 2017, as the DelStar trade name is phased out. The carrying value of the DelStar trade name has been reclassified from Unamortized Intangible Assets to Amortized Intangible assets in the table above as of December 31, 2016.

The following table shows the estimated aggregate amortization expense for the next five years ($ in millions):



For the year ended December 31,	Estimated Amortization Expense
2018	$	20.7
2019	20.6
2020	20.3
2021	20.3
2022	20.2