Once an object needs treatment for anything other than mechanical damage it is likely to be too late.
The best conservation treatment for plastics is preventive conservation as outlined in Environmental requirements and Storage and display. Storing plastics at low temperatures and relative humidities and keeping them away from harmful substances and vapours will however slow down the rate of harmful reactions.
There are no standard interventive processes for plastic materials.
Interventive treatments present risks of further damage to objects due to potential reaction between treatment and object. More damage than good can be done by interventive conservation of plastics.
Do not consider mending plastics or doing other interventive work without the advice of a specialist conservator. Suitable conservators can be found at www.conservationregister.com.