Lubricants are used in plastics mainly to reduce friction between the processing machinery and the plastic materials. Another reason which may not be apparent is the reduction of friction between the molecules of the plastic materials. The lower the friction between internal molecules, the better the performance and the processing properties of the underlying plastic.
Using lubricants can lower power consumption, reduce noise, and help keep wear to a minimum. In some cases, the ability of the plastic material to withstand higher temperatures is also enhanced by the use of lubricants.
There are two types of lubricants that are used with plastics, external lubricants and internal lubricants. External lubricants are generally fatty amides, paraffins, etc. Internal lubricants are generally fatty alcohols, fatty acids, etc.
To check compatibility of lubricants with plastics, manufacturers monitor the changes in physical properties of the plastic material. Metrics observed normally include volume, elongation, hardness, weight, and strength. A pre-determined tolerance level or variation level is referenced. Normally 7% to 10% is a reasonable range. The variations in the physical properties while testing tend to be relatively high when the surrounding temperature is high and conditions are adverse with higher loads.
Selection of lubricants is based on three important criteria. They are as follows:
Chemistry – As outlined in detail in the table below and guidelines below, certain lubricants are compatible with plastic and certain lubricants are not. This is dependent on the chemistry and the chemical structure of the lubricant material. Materials like synthetic hydrocarbons, mineral oils, silicone, and perfluorinated PFAE tend to work well with plastics. At the same time, esters and glycols are not recommended for use with plastics.
If you somehow end up using non-compatible lubricants with plastic, then such lubricants can cause a reduction or even loss of structural integrity and dimensional stability in the plastic. Hence, always follow the compatibility checking process as outlined above.
Sometimes, even additives can be responsible for an undesirable and unintended reaction between plastic and the lubricant. Additives like graphite and molybdenum disulfide can weaken the plastic and create problems with a lubricant that comes in contact with the plastic. Metal part lubricants containing EP additives, corrosion protector, and metal deactivator additives are not recommended for use with plastics.
Viscosity – High viscosity lubricants with ISO VG greater than 100 are not recommended for use with plastics as they can penetrate the plastic material, adversely affecting it. Greases with a consistency of NLGI of 1 or 0 are good as they will help reduce friction and any grease slap (noise induced by grease).
Aging Resistance – Lubricants that age are more likely to attack the plastic material. Hence, it is ideal to use synthetic lubricants in longer-term plastic applications because synthetic lubricants tend to have high aging resistance. Formaldehyde and styrene, which are outgassing plastic byproducts accelerate the lubricant aging process and hence must be kept to a minimum if possible.
Some other guidelines that you can follow while choosing lubricants are:
Silicones and Fluoroethers are generally known to be compatible with almost all plastics
Synthetic hydrocarbons are normally compatible with plastics but not with elastomers
If you plan to use esters, then be very careful when using it with Buna S, Butyl, natural rubber, neoprene, polycarbonate, polyvinyl chloride, polyester, polyphenylene oxide, polystyrene, polysulfone
The melting point of the lubricant should be close to the molding temperature because that will allow the formation of liquid films on the plastic surface when the plastic shape has formed. Too much difference in those temperatures can cause complications.
The effect of lubricant on mechanical and physical properties of the polymer is important because you do not want those properties to deteriorate in the plastic.
Following is a chart displaying the compatibility of synthetic oils with various types of plastics:
|Synthetic Hydrocarbons||Esters & Polyglycols||Silicones (All Types)||Fluorinated Ethers|
|ABS resins||Good||Not Compatible||Good||Good|
|Polyphenylene oxide||Good||Not Compatible||Good||Good|
|Polyethylenes||Use Caution||Use Caution||Good||Good|
Recommended lubricants for plastic
Now that we know which lubricant materials are good for plastic and which ones are not, let us look at what lubricants are best to use with plastic.
Firstly, mineral oils are a great choice because they do not attack plastic and offer good operational as well as economical performance with plastics. They do operate at higher speeds, higher temperatures, and for long duration operations.
If those operational parameters are not what you are looking for, then you can use synthetic lubricants such as PAO hydrocarbon. PAO has great aging resistance and is compatible with most plastics. It is a long-term lubrication solution.
PFAE lubricants are also another option. They have a great balance between wetting and adhesion of plastic surfaces. They are quite popular for use in a very high-temperature environment (500F). For all the benefits that PFAE lubricants offer, they also have a disadvantage in terms of being quite expensive when compared to other lubricant materials.
Lastly, silicone lubricants are also quite popular, especially in low load applications. They have great compatibility with plastic and can operate in a wide temperature range of -90F to 425F.
Montan Waxes are great to use with engineering plastics like PVC, polyester, polyamide, etc. Montan Waxes offer high effectiveness even at low concentration levels. There is no product loss and it is non-blooming.
Polyolefin Waxes are used as an external lubricant with PVC. They provide great lubrication as well as aesthetic appearance, which is especially important in transparent PVC applications like food boxes and fruit boxes seen at supermarkets. Polyolefin waxes are also used as internal lubricants in PO compounds and masterbatch applications.
Amide waxes are also used quite a lot with PO and PVC for internal as well as external lubrication. The big advantage of amide waxes is that they are compliant with regulations related to food contact articles.