Developing our working-together-from-anywhere initiatives to enable our global workforce to collaborate in new ways, increasing the number of females and minorities on our senior leadership team to 35%, creating and filling a diversity, and inclusion leadership position to further strengthen our commitment to equality for everyone, and developing a platform for small groups to talk openly about biases, belief systems, and the importance of valuing different perspectives.
Despite the continued challenges created by the pandemic and further lockdowns across much of Europe, our fourth-quarter adjusted EBITDA margin expanded 230 basis points to 17.4% with adjusted EBITDA of $48.1 million, an increase of 11% year over year.
With approximately $230 million of cash on the balance sheet at the end of the quarter, an untapped revolver, and our relentless focus on cash generation, we're well positioned to consider additional bolt-on acquisition opportunities.
And third, we have made significant progress over the last 18 months, stabilizing our financial results and evolving our portfolio toward more profitable businesses in higher-growth markets to improve cash flow return on investment.
Our nimble response to the COVID pandemic and robust cost mitigation initiatives, together with our portfolio reshaping actions, enabled our company to achieve flat year-over-year adjusted EBITDA despite a nearly 11% decline in sales during 2020.
This represents an adjusted EBITDA margin expansion of 170 basis points for the full year, a tremendous feat for the EnPro team amid 2020 macroeconomic challenges.
With these actions, we anticipate our heavy-duty truck business annual sales will range from $125 million to $175 million, reducing the percentage of our total sales in trucking from the mid-20s to the mid-teens.
This resulted in a 30% increase in backlog mainly driven by an increased order flow from heightened demand for clean room services.
Over the past year, the business has maintained its high-profit margins while increasing revenue approximately 40% driven by continued demand for advanced node semiconductor chip.
In the fourth quarter, sales of $276 million decreased 3.7% year over year, reflecting weakness in oil and gas, general industrial, and aerospace markets.
Excluding the impact of foreign exchange translation, and sales from acquired and divested businesses, sales for the quarter declined 1.6% compared to the fourth quarter of 2019.
On a sequential basis, sales in the fourth quarter increased 2.9% over the third quarter in markets where we saw the greatest sequential sales improvement included general industrial, automotive, power generation, and food and pharma.
Gross profit margin of 37.5% increased 330 basis points versus the prior-year period driven by the benefit of divesting low-margin businesses as well as initiatives supported by the EnPro Capability Center, including supply chain and other companywide cost reduction programs.
The year-over-year improvement in gross profit margin was achieved despite a $3 million amortization of acquisition-related inventory write-up in the fourth quarter of 2020.
Adjusted EBITDA of $48.1 million increased 11.1% over the prior-year period as a result of strategic acquisitions and previously announced cost reductions taken across the company in response to COVID.
Adjusted EBITDA margin of 17.4% increased approximately 230 basis points compared to the fourth quarter of 2019.
Corporate expenses for the quarter were $10.6 million, a decline of 2.8% compared to the prior-year period.
Adjusted income from continuing operations attributable to EnPro Industries was $25.4 million, an increase of 27% compared to the fourth quarter of 2019.
Adjusted diluted earnings per share of $1.24 increased 27.8% compared to the prior-year period.
Amortization of acquisition-related intangible assets in the fourth quarter was $10.9 million compared to $11.1 million in the prior-year period.
We have also updated our estimated normalized tax rate used in determining adjusted net income, and adjusted earnings per share to 30% from the previously used normalized rate of 33%.
In the fourth quarter, we recognized environmental charges of $22 million, which led to our GAAP net loss in the fourth quarter.
The responsibility for these matters was contributed to EnPro at the time of the 2002 spinoff from Goodrich Corporation.
Cash outlays for all environmental matters were $33.8 million in 2020.
For 2021, we expect environmental cash payments to decline to approximately $13 million.
Sealing Technologies, which includes Garlock, STEMCO, and the Technetics sealing business, had sales of $154.7 million in the fourth quarter.
The year-over-year decline of 11.3% was due to softer demand in general industrial and aerospace markets, offset in part by stronger performance in food and pharma and heavy-duty truck markets.
Excluding the impact of foreign exchange translation and sales from acquired and divested businesses, sales decreased 2.9% versus the prior-year period.
On a sequential basis, sales in the fourth quarter decreased 2%.
For the fourth quarter, adjusted segment EBITDA increased 5.4% to $34.9 million despite the decline in sales.
And adjusted segment EBITDA margin expanded 360 basis points to 22.6%.
Excluding the impact of foreign exchange translation, acquisitions and divestitures, adjusted segment EBITDA increased 10.6% compared to the prior-year period.
Fourth-quarter sales of $49.9 million increased 27.3% driven primarily by the acquisition of Alluxa and continued strength in the balance of the segment.
Excluding the impact of foreign exchange translation and sales from acquired businesses, sales increased 10.7% versus the prior-year period.
On a sequential basis, fourth-quarter sales increased by approximately 12% from the third quarter driven by the acquisition of Alluxa at the end of October.
For the fourth quarter, adjusted segment EBITDA increased 57.1% to $15.4 million, and adjusted segment EBITDA margin expanded 590 basis points to 30.9% driven primarily by the Alluxa acquisition and growth in the balance of the segment.
Excluding the impact of acquisitions, divestitures, and foreign exchange translation, adjusted segment EBITDA increased 10.2% compared to the prior-year period.
In Engineered Materials, which consists of GGB and CPI, fourth-quarter sales of $73.6 million decreased 2.5% compared to the prior year, primarily due to weakness in oil and gas, general industrial, and petrochemical markets, partially offset by strength in the automotive and power generation markets.
Excluding the impact of foreign exchange translation, sales for the quarter decreased 5.6%.
Sequentially, sales increased approximately 9% as we saw demand rebound in automotive and general industrial markets.
For the fourth quarter, adjusted segment EBITDA decreased 2.6% and adjusted segment EBITDA margin of 15.5% was flat versus the prior-year period.
Excluding the impact of foreign exchange translation, adjusted segment EBITDA decreased 7.7% compared to the prior-year period.
We ended the quarter with cash of $230 million and had full availability of our $400 million revolver, plus $11 million in outstanding letters of credit.
At the end of December, our net debt to adjusted EBITDA ratio was approximately 1.6 times.
During the fourth quarter, we financed the Alluxa acquisition through a combination of $238 million of cash, and rollover equity from Alluxa executives equating to 7% of the acquisition price.
2020 free cash flow of $39.3 million was down from $109.2 million in the prior year, primarily driven by higher 2020 payments related to environmental settlements and a third-quarter legal settlement, both of which we discussed on our third-quarter earnings call as well as significantly higher year-over-year tax payments, resulting largely from the gain on the sale of Fairbanks Morse.
Excluding environmental and legal settlements as well as tax payments in both years, free cash flow increased 12% from the prior year.
During the fourth quarter, we paid a $0.26 per share quarterly dividend, totaling $5.5 million.
Last week, our board of directors approved a 4% increase in the quarterly dividend from $0.26 per share to $0.27 per share.
Under this authorization, we may repurchase up to $50 million in shares, providing us with the flexibility to return capital to shareholders, subject to balance sheet and growth investment considerations.
As shown on the slide, on a full-year basis, our 2020 pro forma sales are $983 million or 8.5% lower than reported sales.
2020 pro forma adjusted EBITDA is $167.5 million or relatively flat with reported adjusted EBITDA, resulting in a net adjusted EBITDA margin increase of approximately 130 basis points to 17%.
We expect 2021 adjusted EBITDA to be in the range of $178 million to $188 million on sales growth of 6% to 10% over 2020 pro forma sales of $983 million.
We expect adjusted diluted earnings per share from continuing operations to be in the range of $4.32 to $4.66.

Summary:
In the fourth quarter, sales of $276 million decreased 3.7% year over year, reflecting weakness in oil and gas, general industrial, and aerospace markets.
Adjusted diluted earnings per share of $1.24 increased 27.8% compared to the prior-year period.
We expect adjusted diluted earnings per share from continuing operations to be in the range of $4.32 to $4.66.