There's always a difference in quality with these types of...

There's always a difference in quality with these types of products. If the cheaper material is heavier I'll bet that its because the material quality is inferior in some way, that you, I or the Bunnings part-time assistant you spoke don't appreciate.

I can give you 2 good analogies that I am very familiar with:

1. Bikes. Cheaper bikes are always heavier than more expensive bikes. This is because they need to use thicker material (whether steel, aluminium or carbon) to compensate for the lower grade material they are using to achieve the same frame strength. More expensive bike will be stronger and stiffer than cheaper bikes and weigh a lot less - because of the quality of the material used.

2. Wetsuits. The more expensive ones are thinner because the offer more flexibility but the rubber used needs to be stronger.

So for shade cloth, there will no doubt be a difference in quality and the fact that GP shade cloth is thinner is suggestive of that. There are many grades of HDPE, HDPE is not a single plastic its a class of plastics defined by the underlying chemical structure but a whole range of additives may be used to dramatically change the characteristics of a particular HDPE grade. This might be for example to increase the tensile strength, tear resistance, or even the UV resistance (either resistance of the material to degradation due to sun exposure - there will no doubt be a specification that provides a degradation curve, or else the UV resistance to sunlight penetration through the shade cloth, which will probably be related to the mesh gauge. Then there is also the quality of the stitching of the edgework using different grades of cotton or nylon stitching to resist breakdown over time due to the regular flapping cycle a shadecloth will be subjected to due to the wind.

Cheers, Sharks