Single pack primer undercoat for plastic?

[blackrose]

I need to prime a couple of bits of plastic trim before painting them. I had a can of Halfords grey plastic spray primer hanging around for years unused but I had a clearout a few months ago and sods law I must have chucked it out because I can't find it.

 

I don't want to buy more so can I use Hemple Primer Undercoat as I've got lots of that? It says it's suitable for keyed GRP so if I key my plastic parts would I be able to use it?

 

https://www.hempel.co.uk/en-GB/yacht/diy/products/hempels-primer-undercoat-13201

 

The only difference I see is that GRP is a thermoset plastic (I think?) while my parts are probably made of PVC or some other thermoplastic.

 

 

 

Edited August 5, 2020 by blackrose

[matty40s]

We use an etch primer on plastics.

[blackrose]

16 minutes ago, matty40s said:

We use an etch primer on plastics.

I've got some 2 part etch primer designed for aluminium but that seems a bit like overkill?

[WotEver]

2 hours ago, blackrose said:

The only difference I see is that GRP is a thermoset plastic (I think?) while my parts are probably made of PVC or some other thermoplastic.

Close. GRP is glass strands encapsulated in a thermosetting epoxy polymer. Heat it and it won’t melt. Plastic on the other hand, will.
 

GRP is also pretty non-reactive to most substances whereas plastic isn’t, depending on the type. Avoid anything with Acetone in it if you’re painting plastic. 

Hemple is a white-spirit based paint so will probably be ok. @Dr Bob might know. 

[Dr Bob]

8 minutes ago, WotEver said:

Close. GRP is glass strands encapsulated in a thermosetting epoxy polymer. Heat it and it won’t melt. Plastic on the other hand, will.
 

GRP is also pretty non-reactive to most substances whereas plastic isn’t, depending on the type. Avoid anything with Acetone in it if you’re painting plastic. 

Hemple is a white-spirit based paint so will probably be ok. @Dr Bob might know. 

GRP actually stands for glass reinforced polyester. GRP is therefore not a plastic and is not too difficult to bond to as the material has lots of oxygen molecules in it which can bond strongly to other things. Similarly with epoxies which again has 'active' groups as long as it abraided. Plastic on the other hand is a difficult issue. If it's Polyethylene or Polypropylene then it is very difficult to bond to. The material is all carbon and hydrogen atoms which don't allow much to stick. They are very hydrophobic so hate oxygen - which is what most adhesives use. Most adhesives tend to be hydrophilic. If it's PVC then it's easier as the Cl atom helps increase its hydrophilic nature.

I seriously doubt any primer will work with PE or PP (polyolefins) other than flame treatment but maybe there is a proprietary primer for PVC or nylon. A primer for use on GRP used on polyolefis will not be any use.

[blackrose]

21 hours ago, WotEver said:

Close. GRP is glass strands encapsulated in a thermosetting epoxy polymer. Heat it and it won’t melt. Plastic on the other hand, will.
 

 

I thought that's what I'd said? GRP is a thermoset plastic and PVC is a thermoplastic. The term polymer and plastic can be used interchangeably. Plastic is just a finished polymer.

Edited August 6, 2020 by blackrose

[blackrose]

20 hours ago, Dr Bob said:

GRP actually stands for glass reinforced polyester. GRP is therefore not a plastic

 

 

I have to disagree. Since polyester is a polymer then GRP is a plastic in the broader sense. 

 

https://www.plasticon.co.uk/composites/glass-reinforced-plastics

 

https://www.hambleside-danelaw.co.uk/wp-content/uploads/2018/07/Zenon-what-is-GRP.pdf

 

https://en.wikipedia.org/wiki/Polyester

 

Anyway, the single pack Hemple primer stuck to my keyed PVC parts fine.

Edited August 6, 2020 by blackrose

[Dr Bob]

1 hour ago, blackrose said:

 

I have to disagree. Since polyester is a polymer then GRP is a plastic in the broader sense. 

 

https://www.plasticon.co.uk/composites/glass-reinforced-plastics

 

https://www.hambleside-danelaw.co.uk/wp-content/uploads/2018/07/Zenon-what-is-GRP.pdf

 

https://en.wikipedia.org/wiki/Polyester

 

Anyway, the single pack Hemple primer stuck to my keyed PVC parts fine.

No, you are confusing Plastic and Polymer. GRP is made from polymers and glass but the polymers are crosslinked during cure and so are no longer plastic.

Plastics are mouldable, ie they melt and can be formed into new shapes.

Polyester is a plastic. Unsaturated polyester is a plastic but once crosslinked with glass becomes a 3 d network which does not melt and is not mouldable therefore is no longer a plastic. You are also confusing polyester with unsaturated polyester. Big difference.

People who call GRP glass reinforced plastics are technically wrong. You will notice Plasticon like to call it plastic as that ties into its branding. Most people do not know the difference between plastics and polymers and GRP has morphed into an acronym where the 'P' is plastic. In no way can a thermoset be called a plastic unless it is maleable and mouldable - which they almost certainly aren't - the crosslinking stops it.

GRP(from unsaturated polyester or epoxy) is a thermoset whereas the plastics are thermoplastics.....big difference. You can get glass filled nylon or glass filled PP but these are not normally called GRP - as that name is usually reserved for thermosets made from unsaturated Polyester or Epoxy.

As I mentioned earlier, it is a lot easier to stick things to PVC as it is not as hydrophobic as Polyolefins. It is very difficult to stick things to Polyethylene or Polypropylene - the worlds most common plastics.

[Dr Bob]

2 hours ago, blackrose said:

I thought that's what I'd said? GRP is a thermoset plastic and PVC is a thermoplastic. The term polymer and plastic can be used interchangeably. Plastic is just a finished polymer.

No they cant.

See my response above. A plastic is a material that melts or can be moulded. It is a subset of Polymers.

Polymers can either be thermosets or thermoplastics. There is no such thing as thermoset plastic.

Plastic is not just a finished polymer. Plastic is a specific sub group of materials classed as polymers.

I do have a Ph.D in polymer chemistry.

[Tony Brooks]

I suspect this discussion hinges upon who is having it. I am sure DrB is 100% correct but many engineers, and I suspect the more technical person on the street talk of thermoplastics and thermosetting plastics and everyone knows exactly what they are talking about.

[matty40s]

10 hours ago, Dr Bob said:

 

People who call GRP glass reinforced plastics are technically wrong.

Can we still call them yoghurt pots ?

[Dr Bob]

1 hour ago, Tony Brooks said:

I suspect this discussion hinges upon who is having it. I am sure DrB is 100% correct but many engineers, and I suspect the more technical person on the street talk of thermoplastics and thermosetting plastics and everyone knows exactly what they are talking about.

You are right of course...its just the way the English language is being changed.

One of the downsides is the OPs original question on primers....ie if you assume GRP is a plastic then how do you differentiate between primers that work. Another area where the miss use of the word plastic will give big problems is the recycling of plastics. Plastics by definition can be melted/softened and reshaped so it is possible to mechanically or chemically recycle them. GRP or epoxy laminates cannot be mechanically recycled as they cannot be reshaped.

Please though everyone, dont confuse polymers with plastics. They are not interchangeable. Consider the pyramids in Egypt. Some clever peeps thinks the blocks that make the pyramids are man made using inorganic polymers (ie complex silicate/aluminate polymers) - quite easy to cast huge blocks if you have the right chemicals. In no way would anyone think these were plastics.

[dmr]

I suspect language changes and evolves and not always for the better, just look at the word solution, that must have chemists turning in their graves. I did like the plumbers van that I saw sign-written as "liquid solutions".

I suspect a useful thing dissolved in something else could be a "solution solution".

 

But anyway, I reckon plastic is now any solid material that is not metal or wood. Historically is plastic not a property rather than a chemical definition? I can bend a bit of steel till it reaches its plastic limit? Was plastic named because it displayed good plasticity??? And if you formulate a brittle plastic maybe its no longer plastic?

 

............Dave