What is Intrinsic Viscosity (IV)?
Polymers are made up of small building blocks called monomers. When these monomers are linked together, they form a polymer. Intrinsic viscosity is a means of measuring how many of the monomers are actually locked up in the polymer chain – or in short how long the polymer chain is. The higher the IV, the longer the polymer chain, the higher the molecular mass is.
Polyethylene terapthlate (PET) is a polymer used in fibre production for both continuous (endless) & staple (cut) fibre:
The physical and chemical behaviour of PET polymer, the PET fibre and subsequently the PET nonwoven is in large part influenced by how many of the monomer units are linked together in the polymer chain. How many remain linked during processing and subsequent exposure during an intended application will influence the durability of the geotextile. A general rule of thumb is that the longer the polymer chain (more monomers linked), the higher the IV, the stronger, more durable and tougher the fibre and subsequently the nonwoven will be.
Traditionally a 0.64 IV PET polymer (commonly called textile grade) is used in fibre spinning, both in continuous filament (CF) and staple fibre (SF) production. CF and SF production is the precursor to nonwoven production, and as such the fibre (and polymer) imparts characteristics to the final nonwoven fabric.
Generally the higher the polymer IV the more technically difficult it is to form the fibre. Lowered production rates are common with increases in IV, hence making it more difficult to produce fibres at economical rates. Furthermore, polymer with higher molecular mass (IV), commonly called bottle grade (IV >0.84) is subject to an additional polymer manufacturing step (boosting IV and removing residual reactants & monomers) and this reflects in the cost of “virgin” polymer. This is really a case of you pay for what you get when it comes to strength and durability.
To improve the performance of the nonwoven geotextile, careful selection of technology, process equipment and optimised production conditions, are absolutely necessary to achieve an optimum nonwoven outcome.
So why recycled Bottle grade PET into technical textiles?
The fundamental driver for the use of recycled PET bottle polymer into the highly technical geotextile end use lies in that:
- PET polymer in itself has desirable properties – it forms a high tensile, semi-crystalline fibre, with a high melting point (>260°C), is inherently stronger than most common polyolefin fibres “out of the box”
- Bottle polymer has a high IV (considerably higher than virgin textile grade), promoting strength even more when compared to textile grade
- Polymer in bottle form is very consistent, the bottle production process dictates a very narrow polymer and process window and as such the polymer is subjected to extremely tight manufacturing quality control
- Food contact during its primary use makes this grade highly controlled and regulated, ensuring even more consistency
- Uniform bottle polymer “characteristics” offer predictable outcomes for subsequent processing and application
- In short this means that if you take any injection stretch blow moulded carbonated beverage bottle (ISBM) and analyse its polymer, it will only be PET (not another polymer type) and it will exhibit a very consistent IV and hence molecular makeup.
Additives – what do they do, and why are they needed?
Additives are “other” chemicals that are added to impart certain desired characteristics to the polymer. These are characteristics which are not necessarily imparted to the final material by the polymer alone. Normally additives are added to:
- Add colour (most common)
- Prevent polymer break down (reduction in IV) during the conversion process (in the molten phase)
- Impart long term polymer stability against heat, UV and other environmental exposures. All of this in aid of retaining physical characteristics over an extended period of time in the final application
- Makes the conversion process possible, easier, faster and more stable
- Impart special properties like anti-block, improved slip, friction modifiers, hardening agents, clarifiers, nucleators (influence the crystalline structure in semi-crystalline polymers)
Simply adding more and more additives to drive a specific property will without exception influence other desirable polymer characteristics in a negative manner. This necessitates additive formulations that target an optimum blend of various additives and concentration levels to achieve a balanced outcome. Additives may exhibit synergistic (outcome of the sum of two parts is larger than the sum alone) or depleting (cancel each other’s efficacy), or have no impact on each other’s outcome, the latter being very unusual. The need for additives to impart modified performance is dependent on the polymer type. Certain polymers in their unaltered state will exhibit better performance in certain areas than others. Such is this that some polymers require much lower additive dosing or even no additive at all to match another polymer that is heavily dependent on an additive to impart this property.
bidim Geotextile Additives
Additives are added to the PET bottle grade (high IV) polymer during the bidim production process in order to:
- Improve chemical resistance
- Improve fibre tenacity
- Control fibre shrinkage
- Impart improved UV resistance
- Impart improved durability
- Improve processability
- Impart colour – reduce glare
In conclusion, high IV PET promotes long term thermal stability, low reactivity and high strength – hence the success of rPET bottle recyclate in technical textile applications. The ability to process the high IV polymer into a useable fibre is what sets the Bidim process apart from other technologies. It’s ability to produce large diameter PET fibres, with high strength (tenacity) is what makes the Bidim process unique when compared to off the shelf technologies, offering a technological competitive advantage over other technologies.