Papelão vs. cartão
When translating "cardboard", the choice between papelão and cartão depends heavily on the variation of Portuguese being spoken. While both refer to paper-based materials, papelão is the standard term for corrugated cardboard packaging in Brazil, whereas cartão is the preferred term for the same material in Portugal. In Brazil, cartão is almost exclusively reserved for thinner cardstock, such as credit cards or business cards.
Papelão
A2This is the primary word used in Brazil to describe thick, corrugated cardboard used for shipping boxes and heavy packaging. In Portugal, papelão is understood as the raw material, but it is less commonly used in everyday conversation to describe boxes compared to Brazil.
Nós precisamos de muitas caixas de papelão para a mudança.
(We need many cardboard boxes for the move.)
O gato adora dormir em cima de pedaços de papelão.
(The cat loves sleeping on top of pieces of cardboard.)
Por favor, coloque o papelão na lixeira azul para reciclagem.
(Please put the cardboard in the blue bin for recycling.)
A chuva destruiu a caixa de papelão que estava no quintal.
(The rain destroyed the cardboard box that was in the backyard.)
Cartão
A1In Portugal, cartão is the standard word for cardboard boxes and packaging materials. However, in Brazil, cartão generally refers to a small "card" (like a credit card, greeting card, or business card) or thinner cardstock, not the thick material found in shipping boxes.
A encomenda chegou numa caixa de cartão enorme.
(The order arrived in a huge cardboard box (Portugal context).)
Estes sapatos vieram numa caixa de cartão resistente.
(These shoes came in a sturdy cardboard box (Portugal context).)
É proibido deitar caixas de cartão no lixo orgânico.
(It is forbidden to throw cardboard boxes in the organic trash (Portugal context).)
Vamos fazer um castelo usando rolos de cartão.
(Let's make a castle using cardboard rolls (Portugal context).)
Summary
To summarize, if you are referring to a brown shipping box, use papelão if you are in Brazil and cartão if you are in Portugal. While papelão exists in Portugal, usage favors cartão for everyday objects. Conversely, using cartão to mean a heavy box in Brazil may cause confusion, as Brazilians will think you are referring to a credit card or a business card.