COS was $33.3 billion in 2020 , compared with $36.7 billion in 2019 . COS decreased in 2020 primarily due to lower feedstock and other raw material costs, decreased sales volume and lower planned maintenance turnaround costs, which were partially offset by higher performance-based compensation costs. Operating rates declined significantly in the second quarter of 2020 , as the Company temporarily idled certain manufacturing facilities and selectively adjusted operating rates at other facilities to balance production to demand in response to the COVID-19 pandemic. These facilities returned to more normalized operating rates in the third quarter of 2020 . Overall, operating rates increased in the third and fourth quarters of 2020 . In 2019, COS also included $75 million of transaction-related costs resulting from the separation from DowDuPont (related to Corporate) and $399 million of environmental charges related to Packaging & Specialty Plastics ($5 million), Industrial Intermediates & Infrastructure ($8 million), Performance Materials & Coatings ($50 million) and Corporate ($336 million). COS as a percentage of sales was 86.5 percent in 2020 compared with 85.3 percent in 2019. Research and Development Expenses Research and development ("R&D") expenses were $857 million in 2021 , compared with $768 million in 2020 and $765 million in 2019 . R&D expenses in 2021 increased compared with 2020 primarily due to increased performance-based compensation costs and fringe benefit expenses driven by stock market increases compared with the same period last year. R&D expenses in 2020 increased compared with 2019 primarily due to increased performance-based compensation costs which were partially offset by cost reductions. Selling, General and Administrative Expenses Selling, general and administrative ("SG&A") expenses were $1,645 million in 2021 , compared with $1,471 million in 2020 and $1,590 million and $1,585 million for Dow Inc. and TDCC, respectively, in 2019 . SG&A expenses in 2021 increased primarily due to increased performance-based compensation costs and fringe benefit expenses driven by stock market increases compared with the same period last year. SG&A expenses in 2020 decreased compared to 2019 primarily due to cost reductions which were partially offset by increased performance-based compensation costs. In 2020, SG&A was also favorably impacted by the recovery of legal costs related to the Nova Chemicals Corporation ("Nova") ethylene asset matter and the reversal of a bad debt reserve related to an arbitration judgment. See Note 16 to the Consolidated Financial Statements for additional information on the Nova litigation matters. Amortization of Intangibles Amortization of intangibles was $388 million in 2021 , compared with $401 million in 2020 and $419 million in 2019 . Amortization of intangibles decreased primarily due to certain intangible assets becoming fully amortized. See Note 13 to the Consolidated Financial Statements for additional information on intangible assets. Restructuring, Goodwill Impairment and Asset Related Charges - Net Restructuring, goodwill impairment and asset related charges - net were $6 million in 2021 , $708 million in 2020 and $3,219 million in 2019 . 2020 Restructuring Program On September 29, 2020, Dow Inc.'s Board approved restructuring actions to achieve the Company's structural cost improvement initiatives in response to the continued economic impact from the COVID-19 pandemic. The restructuring program was designed to reduce structural costs and enable the Company to further enhance competitiveness while the COVID-19 economic recovery gained traction. These actions were substantially complete by the end of 2021, except for certain cash payments expected to be made in 2022. In 2020, the Company recorded pretax restructuring charges of $573 million, consisting of severance and related benefit costs of $297 million , asset write-downs and write-offs of $196 million and costs associated with exit and disposal activities of $80 million. Restructuring charges by segment were as follows: $11 million in Packaging & Specialty Plastics, $22 million in Industrial Intermediates & Infrastructure, $177 million in Performance Materials & Coatings and $363 million in Corporate. In 2021, the Company recorded pretax restructuring charges of $12 million for asset write-downs and write-offs and $10 million for costs associated with exit and disposal activities. Restructuring charges by segment were as follows: $8 million in Packaging & Specialty Plastics, $1 million in Industrial Intermediates & Infrastructure, $10 million in Performance Materials & Coatings and $3 million in Corporate. In addition, the Company reduced pretax restructuring charges by $10 million for severance and related benefit costs, related to Corporate. 37