OTR stands for Oxygen Transmission Rate, and it is a measure of how much oxygen can pass through a material in a given amount of time. This is an important property to consider in packaging, as it can affect the freshness and shelf life of certain products, particularly those that are sensitive to oxygen. The lower the OTR of a packaging material, the better it is at preventing oxygen from permeating through and affecting the product inside. Therefore, OTR is often used as a quality control measure for packaging materials and is tested using specialized equipment.