Coextrusion, bonding strentgh 4

[manu23]

Hi,

I have a question regarding the bonding strentgh between 2 polymeric materials in coextrusion. I would like to know how the bond strength varies with temperature? Does it decreases until Tg, Tm? or it just drop once reaching them? or combination of both?

Thanks

Manu

 

[Demon3]

Bond strength between two polymers depends on the degree to which the polymers chains can entangle across the boundary between the two polymers. Important factors are:

1. Time of contact, longer contact time in the melt gives time for entanglement and improves bond strength

2. Compatibility of the polymers. As a crude estimation look at the polarity or better the solubility parameters for each polymer (available on the web or in the Polymer Handbook). Polymers of similar polarity will mix and give good bond strength.

3. Molecular weight. High molecular weight will give better bond strength if you give it enough time for entanglement. Lower molecular weigh can diffuse faster so it may give better bond strength for short melt contact times.

4. Temperature. High temperature increases polymer mobility, allowing entanglements to form and thereby improving strength.

5. Impurities. Low molecular weight additives like antioxidants and stearates can decrease strength by collecting at the interface and forming a weak boundary layer (low molecular weight additives cannot entangle and have no strength).

6. Compatibilizer. You can add a block copolmer where each of the two blocks is soluble in one of the polymers to be co-extruded. The compatibilizer can span the boundary and entangle with each phase, improving strength.

7. Use a tie-layer, i.e. a polymer of intermediate polarity co-extruded in a thin layer between the two other polymers to be bonded.

For a book that explains this very well, with data, see...

Polymer Interface and Adhesion by Souheng WU
ISBN 0-8247-1533-0

Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill

 

[Demon3]

Bond strength is expected to decrease at Tg because strength depends on entanglements and at Tg the polymer chains become mobile enough to slip and unentangle over time.

Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill

 

[patprimmer]

Chris

That's the sort of information that makes this site a great place

With your permission I will print it off and use it for my plastics trades students.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[Pud]

Nice one Chris - star indeed for an informative and concise "over the top" post.

I assume similar criteria apply for overmoulding applications?

I hope the OP does the same - star for you!

You have too much time on your hands....

Cheers

Harry

http://www.tynevalleyplastics.co.uk

 

[manu23]

Hi there, thanks for your quick reply.

I know few things that you say. The problem is that so far i have to deal with the material we have, which is i think already well engineered (on a bond strentgh point of view). The only problem is that i have a different application ( ie on temperature). According to what you say the bond strength decreases above Tg. One of my material is PP whose Tg is about -10C. Therefore i have to expect the bond strength to decreases. How weaker will it be at 90C? Does the bond strength decrease lineary up to Tm?

Thanks again

Emmanuel

 

[manu23]

Oh and also, our material undergoes a corona treatment afterward to facilitate adhesion and printing. DO you think it can affect the bond strentgh of the coextrudate?

Cheers

 

[Demon3]

Hi Chaps, I am honored to get such positive feedback from you chaps for whom I have such respect. I would be delighted for you to use it for your students Pat.



Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill

 

[Demon3]

Strength does not decrease linearly between Tg and Tm, the fall-off is far more pronounced than that.

Corona treatment would only affect bonding if you had treated the interface to be bonded so a post treatment on an already bonded specimen should have no effect.

If PP is one of the two materials then what is the other? Knowing that may allow us to help you better.

Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill

 

[manu23]

Thanks Demon3 for all your answers. PP is one of my materials, but i dont think i can tell the 2nd one. However it s got a Tg at about 130C. In our applications, we use devices between 20C and 97C (although it s only for a few seconds at 97). So it is still well below the Tg of one but well above Tg of PP.

 

[Demon3]

Well, if your second, secret polymer is more polar than PP, for example nylon, PET or PBT then you may be able to improve adhesion between the two polymers. Add some maleated PP (Exellor 1020, Epolene wax, Orevac or similar). The polar groups in the maleated PP interact or react with the second polymer to promote adhesion.

Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill

 

[manu23]

Thanks again Chris, I ll try that if i got problem occuring and will let you know how it happens.

Cheers,

manu

 

[Demon3]

If you add filler to the PP it will increase the HDT and keep it stiff when it gets got. It will also reduce creep. Caclium carbonate cheapest and least effective, then talc, then mica then glass fiber. Glass fiber will give you an HDT (softening point) just below the melting point of PP).

Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill