Introduction:
C-PVC is manufactured by chlorinating PVC and shares some of the properties of PVC. Impact modifiers are added to PVC and C-PVC to improve impact strength; especially at lower temperatures.
For U-PVC pipes in India, if fusion is optimized, addition of impact modifiers is not necessary for pipes not exposed to severe weather conditions. However, for profiles having sharp corners and window profiles exposed to solar radiation,
Impact Modifier is necessary.
On the other hand, as seen in the graph, lower elongation at break for C-PVC compound leads to lower impact strength. Impact strength of C- PVC is 30% lower than PVC. Besides, due to more chlorine content it is prone to more thermal and oxidative degradation, leading to UV degradation and in turn loss of impact strength. Hence impact modifiers as well as antioxidants in adequate quantities are essential for C- PVC pipes.
Many processors prefer MBS and CPE or their combination as impact modifiers in C-PVC pipes, instead of All
Acrylic Impact Modifiers (AAM), probably due to easy processability and lower cost.
However, requirement of impact modifiers depend upon end use. Impact modifiers are used for long term use of PVC/C-PVC products especially under cold conditions and outdoor exposure. C-PVC pipes used for conveying hot water from solar heaters or geysers are usually exposed to solar radiation and weathering. Therefore, impact modifiers shall be judiciously selected for C-PVC pipes for indoor and outdoor use.
Type of impact modifiers:
Impact modifiers are elastomeric materials that are partially compatible with PVC/ C-PVC.
Two basic morphologies (structures) are possible for impact modified U-PVC/C-PVC matrix.
1. The particulate structure, and
2. The network structure.
Hence, two classes of impact modifiers exist according to these two morphologies in the form of:
1. Predefined particle size or Core & shell type impact modifiers e.g. AAM, MBS etc.
2. Non predefined particle size impact modifiers. These are thermoplastics that are significantly compatible with PVC/C-PVC e.g. CPE, EVA etc.
All Acrylic Impact Modifiers (AAM or AIM):
It belongs to predefined particle size category and has core and shell structure consisting of -
1. Soft rubbery core based on butyl acrylate / 2-Ethyl Hexyl acrylate having Tg (-) 45 to (-) 60° C, and
2. Hard shell based on methyl methacrylate and styrene that is compatible with PVC and that prevents sticking together and does not melt during processing.
On the contrary, for MBS, core is made of butadiene or butadiene styrene having Tg (-) 70 ° C. The shell is made up of PMMA. Due to butadiene core, it is not preferred for outdoor application.
To obtain weatherability of AAM as well as low temp performance of MBS, Mitsubishi Rayon has commercialized silicon core based modifier.
CPE Impact Modifiers:
CPE belongs to non predefined particle size impact modifiers category. It is prepared by chlorination of HDPE. On chlorination, most of the crystallinity is lost and compatibility with PVC is gained. 30-40% chlorine content CPE in powder form is commercially available having Tg approximately (-) 16 deg C.
Behaviour of CPE during processing and mechanism of impact modification:
Its particle size is similar to PVC/C-PVC that facilitates dry blending.
Besides, CPE melts more or less simultaneously with PVC. As it melts at lower temperature, it surrounds PVC primary particles by a thin elastic membrane and form a network structure. It forms a continuous network inter penetrating the PVC matrix, to a virtually complete molecular blend. This membrane deforms under impact and absorbs the shock.
As the temperature and shear stress increases during processing, the network structure transits into a particular structure. The thin membranes originally surrounding the primary particles tear up and form discrete particles.
Fusion and impact modifiers:
Optimum degree of fusion varies with the type of impact modifier.
For modifiers with predetermined particle size like AAM, a relatively high degree of fusion favors better dispersion of particles and better performance for PVC.
For modifiers with non defined particle size, like CPE, optimum degree of fusion is relatively low for optimum toughness.
Since C-PVC is processed at a temperature 30-35 deg. C higher than PVC, the CPE structure is likely to get converted into particulate structure. And if processed at lower temperature C-PVC fusion may not be adequate to achieve mechanical properties. It is reported that the impact strength by using CPE passes through a maximum.
Some of the salient features of CPE:
1. CPE acts as a coupling agent between fillers and PVC particles. This allows high filler loading without sacrificing physical or optical properties. However, CaCO3 is hardly added to C-PVC, hence this advantage is denied.
2. CPE increases die swell that necessitates modification of tools; otherwise it result in more stretching and in turn more reversion.
3. To achieve same impact strength, higher dosages of CPE are required than AAM. This increases die swell too.
4. CPE has some linear polyethylene segments that act as a lubricant. This necessitates reduction of external lubricants leading to reduction in plate out.
5. CPE having Tg of (-) 16 deg C, reduces Tg of C-PVC pipe while AAM does not reduce Tg.
6. CPE compound runs at lower temperature than acrylics. This also reduces the thermal load.
7. Since CPE contains chlorine, its use in C-PVC adds up to more chlorine, resulting in reduction in thermal stability.
Comparison of AMA and CPE on 1 to 5 scale:
Strength: AMA – 4, CPE – 4.
Rigidity: AMA – 4, CPE – 3.
Thermal stability: AMA – 5, CPE – 4.
Weatherability: AMA – 5, CPE – 4.
Processability: AMA – 5, CPE – 3.
Tg: AAM core (-) 45- (-) 60 deg C, shell 70-120 deg C, CPE (-) 16 deg C.
Some processors use of 50:50 CPE: All acrylic impact modifiers.
