We already know what is happening inside our garden plastic furniture during summer time. But what kind of effect can occur when plastic is exposed to the sun and how to prevent those changes.
The photodegradation can result in a systematic reduction of the molecular weight (depolymerization), or a cross-linking reaction or an oxidation process (called photooxidation). In all cases the changes of the molecular shape appear when plastic is exposed to the sunlight.
The degradation mechanism will lead to the following effects: sticky residues, fine cracking of the surface, chalking, loss of gloss, stress cracking, yellowing discoloration, loss of transparency, loss of impact strength, loss of elongation and tensile strength.
If you don’t know what is chalking, here I come with the explanation. Chalking is a possible effect of degradation. It appears on the plastics or polymer surface as a fine powdery residue. It can be white or another color depending on the filler used. This effect happens when the top layer of organic molecules is destroyed, then the inorganic filler or pigment stay on the surface as dust or chalk.
We know that the photodegradation process in synthetic polymers starts with the participation of photons. The complexity of the following reactions is very high and still are not defined.
The most reactive parts of the polymer chain are the chemical groups which are called the chromophores.
How to stabilize polymers?
The simplest way to get the stabilization of polymers would be protecting the polymer from absorbing the UV radiation using light absorption materials.
On the market, there are several products called UV stabilizers or light stabilizers which are used as additives to polymers.
The most important is to select the UV stabilizer depending on the application of the product, the end-use environment, the polymer base and the processing conditions.
There are four primary types of stabilizers used today: UV screens, UV absorbers, quenchers and free radical scavengers.
UV screens prevent the penetration of UV radiations beyond the surface. This means that if the degradation takes place, the only affected area is the thin surface layer.
The most important UV screen is carbon black which is the most effective stabilizer for most common polymers (e.g.: polyethylene). Its effectiveness depends on its type, size of particles and degree of dispersion within the polymer matrix.
Moreover, carbon black is very efficient as a weathering stabilizer, not only for its ability to screen the polymer from UV radiations but also for its ability to trap radicals formed during the photooxidation.
UV absorbers work by absorbing the UV radiations and transfering them into harmless infrared radiation or into thermal energy. But how to chose the right one?
First of all we need to know which are the most harmful UV waves for the polymer and this can varies in each one. Then we need to find the most suitable UV absorber. The easiest way is to choose a universal UV absorber which, ideally, should absorb the entire solar UV.
Next is to choose a UV absorber which won’t affect the color of the part. For example the group of hydroxybenzophenone and hydroxyphenylbenzotriazole are well known as UV stabilizers that have maximum absorbency between 300 and 400nm and are used for neutral and transparent plastics or coatings.
Quenchers provide the energy transfer inside the molecule. The energy from the excited state is transferred to the ground state. The free radical cannot be formed and the further processes of photooxidation won’t appear.
One of the most popular quencher is nickel chelate which drastically reduces the photooxitation process.
Free radical scavengers
The most famous type of free radical scavengers is Hindered Amine Light Stabilizers called HALS. They are scavenging free radical generated by the UV radiation in order to stop the degradation.
HALS are long-term thermal stabilizers but do not absorb any light above 250 nm. The working mechanism is called Denisov Cycle.
This UV stabilizer is commonly used in a wild range of polymers e.g. in polypropylene for automotive applications, in polyolefines, polyethylene and polyurethane.
In order to improve the UV stabilization, stabilizers are often combined and they create a synergistic effect because they all protect in different ways.
Remember that in any case the most important is to select the UV stabilizer according to the application of the product, the end-use environment, the polymer base and the processing conditions.
Simple, isn’t it?