Why Use Treated Pine?
We need to look deep inside and see some principles of how and why primers work.
So, let's look at the basics.
It is all about PVC to start with. PVC stands for Pigment Volume Concentration – that is, how much pigment (the fillers) you have compared to the amount of paint resin (the glue that holds it all together). It's a bit like mixing concrete and the amount of cement to the amount of gravel you have. Too much gravel and not enough cement and you have a weak finished product
However, in paint making there is also more than one type of cement.
One type is “short oil alkyds” (old primer that you all know about). These dry very fast and have been popular in the past at the timber mill end because of this, however these are not the most flexible or the most UV resistant.
LOSP treatment causes problems with the drying of alkyd type paints, which is why a short oil alkyd was chosen as it dries the fastest under these conditions. Reducing the amount of resin and increasing the pigments also helps to speed up the drying of the paint.
The problem with this type of approach is twofold; firstly, you have a less flexible resin in the paint film, and secondly you have less resin to hold all the pigments together. When the UV light hits the resin it structurally breaks it down and then the resin component breaks releasing the pigments it is trying to hold together, resulting in chalking. Don't forget that resin is an expensive part of a paint formula while pigments are a much cheaper component.
Giving you durable timber for a range of projects
What Resene did was to use a medium/long oil alkyd resin that had better UV resistance and flexibility to start with. Then we put in a medium level of pigment so that the PVC of the paint was mid-range. The advantage of this is that you have more glue holding the paint film together making the primer more UV resistant and more flexible than pigment rich resin poor predecessors.
This however makes the paint more expensive and slower to dry compared to the highly filled cheaper short oil primers as the longer the oil resin the longer it takes to dry.
Of course, you could overcome this by increasing the pigment loading again, however this compromises the strength and flexibility of the primer resulting in a poorer quality product with less weather resistance etc.
Formulation of paints is a very technical process and many things have to be considered. We have to balance the PVC, resin type and many other elements including the substrate (in this case Pine that is treated with LOSP and is still fresh).
Hard-wearing pine for your structural or decorative jobs
The end result is a primer from Resene made for ITI that is more expensive and slower drying at the timber mill end. The advantages, however, are significant for the end user as they can enjoy a good quality primer with a resin that has better flexibility, better UV resistance, good adhesion and top coating adhesion and better water transfer resistance (as there is more resin compared to pigment).
Don’t forget that it is not only “in the now” that is important, as the future is just as important when we think about the life of the paint. All alkyd resin paints dry by oxidation, that is, they harden in reaction to air (oxygen). This process never stops and as time goes by the alkyd paint becomes harder and harder and less flexible. We need some movement in the paint film to allow for the movement of the timber under the paint.
Short oil alkyds, while fast drying become brittle much quicker than medium/long oil alkyds. If they are highly filled, then the cohesive strength is weaker and they are prone to failure much faster than the longer oil alkyds. So, we also developed our primer to be more stable and have a longer life and be slower to develop brittleness, giving the end customer a longer life of their entire paint system.