By far, the most popular option is flexible polyurethane foam (PUR) which is divided into high resilient “classic” foam and viscoelastic “memory” foam. High resilient foam (HR) is what we are used to and by far the most common in sleep ergonomic products, even though memory foam has also a widely used material in mattresses, and it is now one of the leaders in the pillow world as well.
The difference between “classic foam” aka. “high resilient foam” and memory foam aka. “viscoelastic foam” is that they have different density characteristics and reaction to temperature. Classic foam will regain original shape instantly once pressure is removed and support with a constant amount of pressure relief over time. Temperature does not have a significant effect on this function using high resilient foam. Memory foam however, will in comparison give way when pressurized.
Consequently not be as affective providing support which is why Royal Rest has cores made of classic high resilient foam in all pillows, no matter model. As memory foam will cover larger area of the body, due to the body sinking down into the material, it is scientifically established that it preserves body heat in a larger degree than other foam materials. Then it becomes even more important that the material is ventilated to provide for a healthy and sound sleep and that the cores are firm to provide support. Another aspect is that memory foam material reacts easily to temperature and will become firmer the colder it gets and softer the warmer it gets.
To conclude, when it comes to sleep ergonomics, we know that it is not the material that makes a significant difference regarding positioning and aches or pains, choice of material is a matter of preference, it is within the design of the pillow the solution is provided.