WHICH "K VALUE" S-PVC RESIN IS MOST SUITABLE FOR UPVC INJECTION MOULDED FITTINGS?

Introduction:

Suspension grade PVC is available mostly in K-57 and K- 67 variants. This is because commercially these two grades find maximum utilization by PVC processors. While K- 57 resin is used for calendaring of U PVC sheets and injection moulded fittings, K- 67 resin is used for rigid PVC pipes, profiles and films.

Higher the K value, higher is the porosity. Hence K-67 and higher K value resins are used for plasticized PVC applications.

K -57 grade resin is generally used for injection moulding because; due to its lower chain length (molecular weight) its viscosity is lower than higher K value resins. It has higher die swell than K- 67 resin that helps to fill the die and transfer the shining from the mould surfaces to the product. The die swell and jetting can further be manipulated through addition of processing aid. On the other hand, due to lower die swell in K-67 resin, it finds use in profile manufacturing, where dimensional adherence is more important.

Lower K value resins have lower crystallinity than higher K value resin. Hence theoretically its mechanical properties are lower than K 67 resin. However the difference in tensile strength and modulus of elasticity is marginal.

As per IS 7834: 2003 PVC fittings should have minimum3 mm thickness, for any pressure rating pipe. This means that for 90 x 0.8 MPa pipe the minimum thickness is 4 mm and for 90 x 1.0 MPa pipe the minimum thickness is 5.7mm, but fitting manufacturers will produce minimum 3 mm fitting for all pressure rating pipe, since it is not mentioned in the standard. This aspect needs to be clearly addressed in IS 7834 standard. Otherwise, it only means that the fusion has to be within optimum limits for 3 mm thick fittings.

Due to lower diffusivity, the fusion is relatively difficult in K 67 resin than K 57 resin.

The gloss in K-67 resin is also lesser as compared to K- 57 resin and processing temperature is more which is undesirable in injection moulding due to more shear in the nozzle resulting in splay marks or silver streaks, degradation etc.

More processing temperature not only causes degradation but requires longer cooling time to cool the moulded part. Some people use to put the product in cold water. This prevents crystallization and in turn reduces VST. (Annealing improves thermal & certain mechanical properties)

Viscosity considerations for injection moulding:

For injection moulding process, the consistency of fluxed PVC should be such that it can be smoothly injected through the nozzle and sprue bush into the mould cavity.

K- 57 PVC offers lower viscosity than K- 67 resin and fulfils this requirement.

As per the published information, K- 55 resin is best suited for injection moulded fittings. But it is seldom available and is costly. Hence K- 57 resin is the best compromise. Sometimes under the pretext of better mechanical properties, K- 60 resin is also used whenever cost is favourable or K- 57 resin is not available.

K value vs. Viscosity:

When we buy K-57 resin, the test certificate mentions that the K value of resin is 57 +/- 1.

We accept it blindly and are not able to correlate with practical problems faced during production.

In reality when we buy K-57 resin, we may get PVC with K value of 55.6 (rounded off to 56) to 58.4 (rounded off to 58). How does it matter?

It is well-known that for polymers viscosity is proportional to Molecular weight^3.4.

Since K value is a measure of extent of polymerization for PVC, we can say that its viscosity α (K value)^3.4

Thus for K value of 55.6 the viscosity would be α (55.6)^3.4

And for K value 58.4 the viscosity would be α (58.4)^3.4

This amounts to almost 18% variation in viscosity. This is why, in spite of buying PVC resin from the same supplier and test report mentioning that the K value is 57+/-1, we find processing problems and have to adjust lubricants, especially polar (internal) lubricants. What would happen if K-67 resin is used?

Key to consistent processing is to find the actual K value of incoming PVC in our QA lab. We should then conduct trials to find level of lubrication required for different K value resins and use this data for formulations to get smooth processing. What we are saying is that instead of changing the processing parameters for change in input compound, input compound can be modified to ensure that the process parameters (processing window) remain the same. This would give a great relief to the operator.

K value Vs. VST:

The glass transition temperature of amorphous polymers is a function of its molecular weight

The relationship is expressed as below –

Tg = Tg (inf) – C1 / Xn^0.666

Where,  Xn – is the chain length of the polymer.

             C1 – is the constant

             Tg (inf) – is the glass transition temperature of polymer of infinite chain

For PVC, C1 = 508 and Tg (inf) = 356° K. [356 – 273 = 83 deg C]

If we consider that for K-57 resin, the chain length or degree of polymerization is 750 monomer units, and for K-67 resin, the chain length or degree of polymerization =1000 monomer units,

Then for K-57 resin, Tg = 356 - 506/750^0.666 = 349.81 °K

And for K-67 resin, Tg = 356 - 506/1000^0.666 = 350.89°K

Thus, the theoretical difference in softening point or Tg of K-57 and K-67 resin is 350.9°K-349.8°K = (350.9 – 273) – (349.81- 273)° C = 1 degree C.

However, practical difference would be almost nil, as we add more lubricants to K-67 resin in order to reduce its viscosity and make it injectable. 

Does it mean that one cannot produce fittings from K-67 resin?

We can produce, but since it requires higher temperatures more antidegradants are required to avoid onset of degradation. To reduce viscosity and frictional heat, appropriate lubricants, especially polar lubricants are required. Due to higher temperature and viscosity nozzle diameter needs to be increased. More heat needs to be removed during cooling resulting in higher cycle time.

Therefore, for the best possible processing and mechanical as well as thermal properties K-57 resin is the best solution.