MANIPULATING THE GAP BETWEEN Tb AND Tg OF U-PVC PRODUCTS TO SUIT PERFORMANCE REQUIREMENTS

Introduction:

I prefer to call PVC as Polymer with Versatile Characteristics based on its availability, flexibility in terms of getting different types of products through compounding from same PVC and processability on any type of equipment. This makes it a very versatile plastic, besides it is cheap.

PVC has engineering properties too that finds its use in pipes, profiles, and in construction.

Modulus of elasticity for UPVC is ~ 27500 kg/cm^2 that approaches the modulus of elasticity of engineering plastics like PC (25000 kg/cm^2), Nylon (20200 kg/cm^2), Polyacetal (30000 kg/cm^2), ABS (30000 kg/cm^2) etc.

The mechanical strength of UPVC products comes from optimum fusion during processing. Once we achieve optimum fusion, the next step would be to improve its service life.

Though PVC is most researched polymer, it has certain drawbacks in terms of thermal and UV degradability, lower glass transition temperature (Tg ~ 80°C) and higher brittleness temperature            (Tb ~ 5°C) that can be manipulated through additives to suit performance requirements.

Difficulties can arise if there is lack of -

1.      Knowledge of additives and compounding.

2.      Knowledge of processing that centres around optimising fusion of 65 - 70%

3.      Knowledge of Its drawbacks during service life in terms of limitations in Tb and Tg.

Limitations in service life:

1.      PVC has Tg of 80 deg C, but it gets altered due to additives. For continuous operations during its service life, after considering the safety factors, the maximum service temperature is considered as 55° C.

2.      On the other hand, the lower service temperature after considering the safety factors, the minimum service temperature can be 10° C.

This means practically PVC products work well between Tg & Tb, say between 10-55°C.

Attempts are being made to effectively manipulate the gap between Tb and Tg.

There are four possibilities to alter the band between Tg and Tb:

1.      Reduction in band width,

2.      Shifting the band towards lower temperature side,

3.      Shifting the band towards higher temperature side, and

4.      Widening the band width to improve service conditions latitude.

Reduction of band width:

Some processors add small quantity of plasticizer like DOP to make processing smooth. However, use of DOP reduces Tg @ 3.5 °C per phr that makes the product deform at lower temperature.

Besides, at low level of DOP, say 5 Phr of DOP, the product becomes more rigid due to antiplasticization effect. Though there is an increase in tensile strength and modulus of elasticity, the impact strength drastically reduces. This indicates increase in Tb together with reduction of Tg by 15 deg C. Thus the band width between Tb and Tg will be reduced.

Shifting the band towards higher temperature side:

When miscible polymeric HDT improvers are added, they do increase the Tg based on their individual Tg and % of addition. Such additives can be based on Glutarimide, CPVC, AMS etc. but they impair the impact properties. Change in Tg follows the Fox equation:

1/Tg mix = Xa/Tga + Xb/Tgb

Where, Tgmix, Tga & Tgb are glass transition temperatures of the mixture, polymer “a” and polymer “b”, while Xa and Xb are the weight fraction of polymers “a” and “b” respectively.

Also, when PVC products are annealed, their HDT improves at the cost of Tb. This is why when U-PVC pipe is tested immediately after production; it exhibits lower tensile properties (hoops stress) and more falling weight impact strength. However, when the pipes are conditioned/aged/annealed at 27 °C for say 24 Hrs (IS: 4985), it exhibits more tensile properties and less impact strength.

Shifting the band width towards lower temperature:

To reduce brittleness temperature Tb to sub zero temperatures, we need to add impact modifiers. Various impact modifiers are available depending on whether the product is used indoor (Ex: MBS) or outdoor and the probability of getting hit by hails or flying objects like stones etc, (Ex: AAIM).

For outdoor applications again we have to choose the impact modifiers based on its effect on glass transition temperature, where the ambient temperature is more. CPE for example provide low temperature impact properties as well as fair weathering properties and it is cheaper. But it has Tg of 10-16 deg C, hence it may not remain elastomeric at lower temperature. Its addition will also reduce Tg of the product. In such cases, the band will shift towards lower temperature side.

Widening the band between Tb and Tg:

Ideally, we need to find impact modifiers having Tg more than PVC, whose addition may reduce Tb and increase Tg.

GIYA HUYNH-BA reported that a binary blend (BB) using HMW IMAC (Imidized acrylic polymers) could be added to PVC to increase toughness (impact strength) as well as HDT.

Impact modifiers are elastomers and semi compatible with PVC.

If they are incompatible, sufficient dispersion of rubber in the glassy polymer will not be achieved and hence sufficient adhesion between the phases would not be accomplished.      Such a system has no utility in resisting impact.

If they are too compatible with PVC, then sufficient adhesion would be obtained, but heterogeneity of the system would be reduced. Under such condition, elastomer behaves as a plasticizer (solid plasticizer or plastifiers).

Certain core and shell type of impact modifiers may offer a via media and provide the best combination.

They are graft or cross linked products having –

1.      Soft rubbery core that absorbs the impact shock, and

2.      Hard shell that transfer the impact energy to softer core but remains intact during processing.

They are usually based on methyl methacrylate and styrene that is compatible with PVC.

Atactic PMMA used in shell has Tg more than 100°C. Jager et al pointed out that adding small amounts of atactic -PMMA to PVC may raise the glass transition temperature (Tg) 10 -15°C above the Tg of pure PVC.

Shell thickness of core/shell impact modifier is a single most important factor in toughening PVC:

JUH-SHYONG LEE reported that when the shell thickness is greater than a critical value of 15.8 nm, these core-shell elastomeric particles are able to remain structurally intact and well dispersed within the PVC matrix after melt blending. Too thick shells lose their rubbery nature.

On the other hand, when the shell thickness is less than the critical value of 4.9 nm, too thin a shell layer is simply unable to fully protect and cover the inner rubbery core during vigorous processing conditions, and these core-shell particles tend to connect with one another through the partially exposed core to form a cellular- like structure, thus resulting in poor toughening efficiency.

These elastomeric core and shell particles form a micro particulate rubbery disperse phase in PVC matrix. The tiny rubber spheres absorb the impact shock and dissipate the energy.

Due to its hard shell structure, it does not melt during processing and are largely independent of the amount of shear.

Impact modifiers are not used beyond certain level, say 5-6 phr.

Thus, there is a possibility that a well designed core and shell impact modifier having a shell that has Tg more than PVC and optimum shell thickness may reduce Tb as well as increase Tg, especially for products like PVC fittings having VST marginally less than required 78 deg C.

However, in my opinion, if the shell thickness is much less than 15.8 nm, then during harsh process conditions shells may break up exposing the core and increase in Tg may not be possible. Such a condition may arise if shear is more or instead of K-57 resin someone uses higher K-value resin, say K-67 resin, under the pretext that it will give higher VST of the product.