In 2022, Duke students shredded and melted plastics 1, 2, 4 and 5 into bricks; a $248.84 setup produced blocks holding over 5,000 N in compression tests

On average, each individual in the United States discards about 4.9 pounds of municipal solid waste each day, according to the Environmental Protection Agency's National Overview: Facts and Figures on Materials, Wastes and Recycling, the agency's 2018 national accounting and the latest comprehensive estimate it has published for total MSW generation. That waste stream includes a large share of plastic, much of which is not accepted by curbside recycling; sorting systems struggle with mixed resins and contamination, so a significant portion of what goes into the blue bin still ends up in a landfill. It was this exact frustration that pushed a group of Duke University students to ask a simpler question: instead of hoping a plastic container gets recycled the conventional way, could it just be turned directly into a building material?

The team selected plastics #1, #2, #4, and #5 for testing

That idea was the basis for a project carried out by Bass Connections at Duke University. It was a group of civil engineering, environmental science and economics students working under the guidance of faculty members of the Nicholas School of the Environment and the Pratt School of Engineering. The name of the project was ‘Making Recycled Plastic Bricks with Optimized Accessibility,’ and it was conducted during the 2021-2022 academic year with the aim of testing whether regular household plastic waste can be recycled to produce bricks for construction.


Firstly, they determined types of plastic materials that would serve as the basis for the bricks. Those were plastic types #1 (PET), #2 (HDPE), #4 (LDPE) and #5 (PP), as specified in the project report. Types of plastic were scored on accessibility, environmental benefits, shredability and meltability, and energy consumption. Those four types were chosen as the basis for the bricks. Another type of plastic, plastic #6 (HIPS), was also considered, but it scored lower on the group's overall composite index, accessibility, environmental benefit, and energy use, alongside shredability and meltability and was set aside in favor of the four selected resins.

The full setup cost $248.84 in materials

It is not the choice of plastics alone, but the affordability with which the required processing equipment can be built that sets the project apart. The total price of materials used was $248.84 for a shredding phase, melting phase and molding phase. The list of tools included regular scissors, a guillotine paper trimmer, and a modified credit card shredder for use on plastic bags and films. Melting took place in a toaster oven placed inside a fume hood, while the melted plastic was molded with the help of a regular bread pan lined with parchment paper.

To give some perspective, the team's report compares this project to another one, the Precious Plastic brick-making kit, which in 2016 was priced at around $10,200 for a considerably more sophisticated setup and is still much harder to implement than the one proposed by the team. Moreover, since the plastic feedstock is household waste collected at no cost, the raw material itself is effectively free; it is the $248.84 fixed cost of the shredding, melting, and molding equipment that must be weighed against standard clay or concrete bricks, which typically cost several tenths of a dollar apiece to produce. The plastic bricks also offer several properties over conventional ones: they are thinner and lighter than standard masonry, offer notably better heat insulation, and flex rather than crack.

The bricks held over 5,000 N of compressive force and 37 MPa of tensile strength

However, the true test was the strength of the material. The bricks composed of plastics #1, 2, 4, and 5 had endured more than 5,000 newtons of compressive force without reaching the breaking point; rather, the number of newtons indicated the maximum pressure that the machine used by the group could exert. Additional tensile tests demonstrated the ultimate strength of the brick material as 37 megapascals. Such results, obtained on such a simple apparatus, provided preliminary evidence that homemade plastic bricks can bear meaningful loads under short-duration testing, though, as later sections make clear, this does not yet confirm how the material performs under sustained, real-world structural loads.

HDPE bricks exceeded clay brick strength; PP bricks came close, in a 2022 study

A somewhat industrial and separate approach to a similar concept is provided in the work of Kulkarni et al., published in 2022 in ‘Cleaner Materials’, an Elsevier journal, as described in the article on ScienceDirect. Unlike the previous example, the present research involved only two types of plastic, HDPE and PP, obtained from industrial waste in Maharashtra, India and formed into 190 by 90 by 90 millimeters bricks. The compression test results showed that HDPE bricks had a compressive strength of 11.19 MPa, while PP bricks had a compressive strength of 10.02 MPa, being similar to 10.5 MPa of a conventional first-class clay brick; the compressive strength of the HDPE brick was just slightly higher, while that of the PP brick was slightly lower.

The same study also tested a full wall assembly, finding that the plastic-brick wall sustained a higher ultimate load and failed in shear, whereas a comparable conventional brick wall failed vertically at a lower load. The wall made of plastic bricks had an ultimate load of 197.50 kN before its collapse through shear, while a similar wall of conventional bricks could bear an ultimate load of 153.95 kN before collapsing vertically. In other words, the wall of plastic bricks was stronger than the wall of clay bricks and even failed in a more predictable way.

Long-term durability and scalability remain untested

Strength in one compression test is one thing; long-term durability is another. It must be noted here that the Duke team acknowledged that further research is required to gain insight into how the performance of the brick under continuous heavy load, continuous sun exposure, interactions with other building materials, as well as the effects of fire retardant, will work out. In addition, the above-mentioned study highlighted the fact that any attempt to scale the process beyond the bread pan and toaster oven setup would entail resolving issues such as the uniformity of the melting temperature and the size of the mold, issues not anticipated by the $248.84 setup.

In comparison to the previous project, the Kulkarni research goes into somewhat greater depth since it involves the testing of a complete wall construction and its fire resistance; however, unlike Duke research, it uses one type of waste only rather than the mixture of waste generated in a normal household setting. Taking all this into account, what remains unknown in terms of practical use is whether the plastic bricks will be able to retain their strength under real conditions when it comes to years of exposure and interaction with contaminated plastic waste.