In general all plastics allow the transmission of microwaves; however, not all plastics work well in the microwave. It is really up to the manufacture of the particular container in question to approve it for microwave use since there are other things they have to be considered, such as wall thickness and strength of the container, as well as the maximum temperature that a plastic is able to withstand. Contents of containers may exceed the actual plastic containers temperature resistance leading to damage to the container and potential injury.
Most microwavable plastic food containers are going to be polypropylene. While in general polypropylene is considered to be microwavable, again you would need to check with the manufacture before using in the microwave. LDPE is also considered microwavable but it does not handle high temperatures and is often only used for the lid material.
Many of our manufactures do not recommend HDPE for the microwave and we suggest that you avoid it unless the manufacture has specifically approved it. PET and polycarbonate should not be used for microwaving as they tend to absorb heat. Acrylic and polystyrene are also not recommended.
Caution should be taken when reheating food high in sugar or fat as they can cause damage to microwave safe containers. See microwave manufacturer's instructions for information regarding the use of microwave safe containers in your microwave. Always remove lid or vent containers before heating in microwave. Plastic containers that are microwave safe are not intended for ovens, stovetops, or other browning elements.
Among commonly used plastics for labware
Polymethylpentene and Polysulfone are the most transparent to microwaves. We recommend their use when the chemical and temperature resistance of Polymethylpentene and Polysulfone are compatible with the materials being heated. Flouropolymers can be used when aggressive chemicals such as acids or solvents are being heated; proper venting is essential. Closure threads must be completely disengaged before heating bottles or containers in a microwave.
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