million. Cash flow used by working capital increased in 2019 due primarily to cash used by higher contract assets of $45.2 million, higher inventory of $31.1 million, lower accounts receivables of $2.9 million and lower accrued liabilities and income taxes payable of $12.4 million, partially offset by cash provided by higher contract liabilities of $19.7 million. During 2020, we contributed $15.9 million t o our defined benefit pension plans as compared to $37.3 million in 2019. Decreases in accounts receivable provided $45.6 million and $2.9 million of cash flow in 2020 and 2019, respectively, as compared with cash flow used of $25.4 million in 2018. F or the fourth quarter of 2020 our days' sales outstanding ("DSO") was 69 days as compared to 67 days for 2019 and 72 days for 2018. We have not experienced a significant increase in customer payment defaults in 2020. Decreases in inventory provided $15.3 million of cash flow in 2020, as compared with cash used of $31.1 million and $29.3 million in 2019 and 2018, respectively. The cash used from inventory in 2019 was due to an increase in raw materials and work in process. Inventory turns were 4.1 times at December 31, 2020, as compared with 4.3 times for 2019 and 4.2 times for 2018. Our calculation of inventory turns does not reflect the impact of advanced cash received from our customers. Decreases in contract assets provided $4.3 million of cash flow and decreases in contact liabilities used $34.1 million of cash flow in 2020. Increases in contract assets used $45.2 million of cash flow and increases in contact liabilities provided $19.7 million of cash flow in 2019. Decreases in accounts payable used $22.6 million of cash flow in 2020 compared with cash provided of $24.7 million and $7.6 million in 2019 and 2018, respectively. Increases in accrued liabilities and income taxes payable provided $50.2 million of cash flow in 2020 compared to $12.4 million 2019 and cash used of $15.2 million in 2018. Cash used by investing activities were $41.7 million in 2020, as compared to $33.4 million and $81.5 million in 2019 and 2018, respectively. The increase of cash used in 2020 was primarily due to lower proceeds provided from the disposal of assets during the year of $13.8 million, compared to $42.3 million in 2019. Capital expenditures were $57.4 million, $75.7 million and $84.0 million in 2020, 2019 and 2018, respectively. In 2021, we currently estimate capital expenditures to be between $70 million and $80 million, before consideration of any acquisition activity. Cash provided by financing activities were $147.6 million in 2020 compared to cash flow used of $231.5 million and $173.3 million in 2019 and 2018, respectively. Cash inflows during 2020 resulted primarily from the $498.3 million in net proceeds from the issuance of the senior  notes due October 1, 2030 ("2030 Senior Notes"),  partially offset by a $191.3 million in payments on long-term debt resulting from our partial tender offer of our 2022 Euro Senior Notes, $104.2 million of dividend payments and the repurchase of $15.0 million of our common stock. Cash outflows during 2019 resulted primarily from $105.0 million in payments on long-term debt and $99.6 million of dividend payments. Cash outflows during 2018 resulted primarily from $99.4 million of dividend payments and $60.0 million in payments on long-term debt. On Fe bruary 17, 2021, we announced that we will redeem the outstanding balance of our 2022 Euro Senior Notes on March 19, 2021. We will use the remaining net proceeds from the public offering of the 2030 Senior Notes for the redemption. In 2020 we repurchased 1,057,115 shares of our outstanding common stock for $32.1 million. As of December 31, 2020, we had $113.6 million of remaining capacity under our share repurchase plan previously approved by the Board of Directors. Our cash needs for the next 12 months, including for our estimated 2021 capital expenditures described above, are expected to be lower than those of 2020 due to anticipated benefits from working capital reductions and savings from 2020 Realignment Program, discussed in Note 22 to our consolidated financial statements included in Item 8 of this Annual Report. We believe cash flows from operating activities, combined with availability under our senior credit facility and our existing cash balances, will be sufficient to enable us to meet our cash flow needs for the next 12 months. However, cash flows from operations could be adversely affected by a continued decrease in the rate of general global economic growth and an extended decrease in capital spending of our customers, as well as economic, political and other risks associated with sales of our products, operational factors, competition, regulatory actions, fluctuations in foreign currency exchange rates and fluctuations in interest rates, among other factors. We believe that cash flows from operating activities and our expectation of continuing availability to draw upon our credit agreements are also sufficient to meet our cash flow needs for periods beyond the next 12 months. 45