The following table details the stock-based compensation expense by type of award for the years ended December 31, 2020, 2019 and 2018:

(In millions)	2020		2019		2018
Non-qualified stock options	$	6.8	$	4.5	$	3.6
Restricted stock, service based (“RSUs”)		8.4		6.4		6.1
Restricted stock, performance based (“PRSUs”)		(0.7)		6.2		5.8
Employee stock purchase plan		0.2		0.6		0.5
Stock-based compensation expense	$	14.7	$	17.7	$	16.0
Tax benefit from stock exercised and converted during the period	$	4.5	$	9.1	$	7.8

Non-qualified stock options (“NQOs”) have been granted to our employees and directors under our stock compensation plan. Options granted generally vest over three years and expire ten years from the date of grant.

A summary of option activity under the plan for the three years ended December 31, 2020 is as follows:

	Number of			Weighted-		Remaining
	Options			Average		Contractual Life
	(In millions)			Exercise Price		(in years)
Outstanding at December 31, 2017		1.7		$	31.18		5.19
Options granted		0.2		$	68.15		—
Options exercised		(0.4	)	$	16.06		—
Outstanding at December 31, 2018		1.5		$	38.97		5.20
Options granted		0.2		$	65.56		—
Options exercised		(0.4	)	$	25.95		—
Outstanding at December 31, 2019		1.3		$	47.92		5.60
Options granted		0.5		$	54.82		—
Options exercised		(0.2	)	$	32.18		—
Outstanding at December 31, 2020		1.6		$	51.07		6.04

	Year Ended December 31,
(In millions, except weighted average exercise price)	2020		2019
Aggregate intrinsic value of outstanding options	$	8.3	$	31.8
Aggregate intrinsic value of exercisable options	$	5.7	$	28.0
Total intrinsic value of options exercised	$	2.5	$	22.9
Total number of options exercisable		0.9		0.9
Weighted average exercise price of options exercisable	$	46.29	$	41.00
Total unrecognized compensation cost on non-vested options (a)	$	3.8	$	1.5

We estimated the fair value of stock options at the grant date using the Black-Scholes option pricing model with the following assumptions for the years ended December 31, 2020, 2019 and 2018:

	2020			2019			2018
Risk-free interest rate		0.85	%		2.61	%		2.53	%
Expected option life (in years)		5.96			5.99		5.95
Dividend yield		1.1	%		0.8	%		0.7	%
Volatility		44.35	%		35.05	%		35.64	%
Weighted-average fair value per option granted	$	19.50		$	22.90		$	24.36

The weighted-average expected life is derived from the average midpoint between the vesting and the contractual term and considers the effect of both the inclusion and exclusion of post-vesting cancellations during the ten-year period. Expected volatility is calculated based on a blend of both historic volatility of our common stock and implied volatility of our traded options. We weigh both volatility inputs equally and utilize the average as the volatility input for the Black-Scholes calculation. The risk-free interest rate for the expected term is based on the U.S. Treasury yield curve in effect at the time of grant and corresponding to the expected term.

As of December 31, 2020, a total of 543,391 shares of service based restricted stock units were outstanding, which vest based on years of service under the 2003 and 2013 incentive stock plans. RSUs are granted to key employees, executives, and directors of the Company. The fair value of the RSU is based on the closing market price of the Company’s common stock on the date of grant and is amortized on a straight-line basis over the requisite service period. The stock-based compensation expense recognized is based on an

estimate of shares ultimately expected to vest, and therefore it has been reduced for estimated forfeitures. The total compensation expense related to awards granted to retirement-eligible employees is recognized on the grant date.

The table presented below provides a summary of the Company’s RSU activity for the years ended December 31, 2020, 2019 and 2018:



	RSUs			Weighted-Average
	Number of (In millions)			Fair Value Grant Date
Outstanding at December 31, 2017		0.5		$	36.75
RSUs granted		0.1		$	66.63
RSUs issued		(0.1	)	$	41.46
Outstanding at December 31, 2018		0.5		$	41.65
RSUs granted		0.1		$	68.85
RSUs issued		(0.2	)	$	44.75
Outstanding at December 31, 2019		0.4		$	48.06
RSUs granted		0.2		$	51.51
RSUs issued		(0.1	)	$	51.82
Outstanding at December 31, 2020		0.5		$	47.98

As of December 31, 2020, there was total unrecognized compensation cost related to non-vested RSUs of $8.8 million, which is to be recognized over the remaining vesting period ranging from one year to three years.

As of December 31, 2020, a total of 294,941 shares of performance based restricted stock units were outstanding under the 2003 and 2013 incentive stock plans. The total amount of PRSUs that will ultimately vest is based on the achievement of various financial performance targets set forth by the Company’s Compensation Committee on the date of grant. PRSUs are based on a three-year performance period. The stock-based compensation expense related to awards granted to retirement-eligible employees is expensed on the grant date and is trued up as projections change. The fair value of the PRSU is based on the closing market price of the Company’s common stock on the date of grant and is amortized straight-line over the total three year period. A change in the performance measure expected to be achieved is recorded as an adjustment in the period in which the change occurs.

The table presented below provides a summary, of the Company’s PRSU activity, at original grant amounts, for the years ended December 31, 2020, 2019 and 2018:

				Weighted-
	Number of			Average
	PRSUs			Grant Date
	(In millions)			Fair Value
Outstanding at December 31, 2017		0.4		$	45.35
PRSUs granted		0.1		$	68.15
PRSUs issued		(0.2	)	$	43.96
Outstanding at December 31, 2018		0.3		$	51.75
PRSUs granted		0.1		$	65.56
PRSUs issued		(0.1	)	$	41.71
Outstanding at December 31, 2019		0.3		$	60.48
PRSUs granted		0.1		$	74.74
PRSUs issued		(0.1	)	$	50.50
Outstanding at December 31, 2020		0.3		$	68.77

As of December 31, 2020, there was total unrecognized compensation cost related to non-vested PRSUs of $1.7 million, which is to be recognized over the remaining vesting period ranging from one year to three years. The final amount of compensation cost to be recognized is dependent upon our financial performance.

During 2020, 2019 and 2018 cash received from stock option exercises was $3.3 million, $10.7 million and $5.1 million, respectively. We used $7.7 million, $6.2 million and $5.9 million in cash related to the shares withheld to satisfy employee tax obligations for RSUs and PRSUs converted during the years ended December 31, 2020, 2019 and 2018, respectively.

We classify the cash flows resulting from these tax benefits as financing cash flows. We either issue new shares of our common stock or utilize treasury shares upon the exercise of stock options or the conversion of stock units.

In January 2019, the Compensation Committee of the Board of Directors of Hexcel Corporation adopted, and stockholders subsequently approved in May 2019, an amendment to the Hexcel Corporation 2013 Incentive Stock Plan (the “Plan”) that increased the number of shares of the Company’s common stock authorized for issuance under the Plan by 3,300,000 shares. As of December 31, 2020, an aggregate of 4.0 million shares were authorized for future grant under our stock plan, which covers stock options, RSUs, PRSUs and at the discretion of Hexcel, could result in the issuance of other types of stock-based awards.

The Company offered an ESPP, which allowed for eligible employees to contribute up to 10% of their base earnings, to a maximum of $25,000 in a calendar year, toward the quarterly purchase of our common stock at a purchase price equal to 85% of the fair market value of the common stock. There were 35,000 and 48,236 ESPP shares purchased in 2020 and 2019, respectively. The ESPP was suspended in April 2020 in response to the COVID pandemic.