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Disposable nappies are changing, but not to the naked eye. The change is hidden in the polymers that make up the adhesives in their non-woven materials. Will Nynas specialty oils work in the ever-changing adhesives market?
As one of the main markets for hot melt adhesives, disposable nappies, or diapers as Americans call them, are of great interest to Nynas. This is due to the fact that hot melt adhesives, used in the construction of the non-woven materials used in nappies and other hygiene or medical products, represent a large and important market for Nynas’ specialty oil. It is therefore vital to follow any developing trends within this industry segment and to make sure that the company’s products are fit for purpose. Driven by new polymer technology, the hot melt segment of the adhesives industry is rapidly changing.
To assess how this on-going development may impact future sales of specialty oil, we set up a study to investigate how the company’s plasticisers perform in conjunction with these new polymers. The polymers, metallocene catalysed thermoplastic polyolefins (mPOs), offer major advantages over currently used polymers based on Ziegler-Natta (ZN) catalysis, and sooner or later mPOs are likely to replace most of them. Due to their unique characteristics and low cost, the demand for mPOs is growing rapidly in the adhesives industry.
Metallocene catalysts for polymerisation were invented in 1973 and introduced into the polymer industry during the 1990s. Among their benefits is the ability to polymerise any unsaturated vinyl monomer and also a catalytic activity that is 10 to 100 times higher than ZN catalysts. In addition, they have only one active site, which guarantees uniformity.
Figure 1. Structure of polymers included in the study. (PS=polystyrene)
Due to the single site catalysis, a wider range of structures and narrower molecular weight distribution can be achieved compared to chromium oxide and ZN catalyst systems. This makes it possible to tailor polymers according to the performance requirements for certain applications. Compared with conventional ZN catalysed polymers, mPOs offer more versatile properties, such as toughness, durability, chemical inertness, environmental cleanliness, dielectric strength and clarity.
During the past few years, mPOs have become significant market players and are growing continually in several industrial segments. A dominant polymer in hot melt adhesives is styrene block copolymers (SBCs). However, mPO-based alternatives are penetrating into this segment and have started to replace SBCs. The estimated global demand for SBC and mPO is expected to be nearly equal by 2020.
The aim of our study was to test and compare the performance of Nynas’ and our competitors’ plasticisers (see details in Table 1) in hot melt adhesive formulations based on mPOs and SBC. The test series of the study were carefully selected in order to supply sufficient data and information on the required and most important properties for hot melt adhesives. These properties are: processability, mechanical performance, viscoelastic properties and the material response to exposure to extreme environmental conditions (heat and colour stability). Where it was applicable, the tests carried out were based on industrial standard test methods.
|Property||Unit||Test method ASTM||Nyflex 222B||Nyflex 223||Nyflex 220||Paraffinic technical white oil|
|Density, 15°C||kg/dm3||D 4052||0.895||0.900||0.910||0.874|
|Viscosity, 40°C||mm2/s (cSt)||D 445||100||84||137||105|
|Flash Point, PM||°C||D 93A||214||216||221||260|
|Pour point||°C||D 97||-36||-33||-30||-12|
|Colour, Saybolt||D 156||30||30|
|US FDA § 178.3620||(b)||(c)||(c)||(b)|
mPO hot melt formulations are similar to SBC formulations. The key components are: metallocene polymer (10-20 wt%), tackifier resin (40-60 wt%), plasticiser (20-30 wt%), wax (5-20 wt%), additives (0.3-5%) and stabilisers (0.3-5%). In order to evaluate the plasticizer oils and to minimise the influence of the other components on the performance of the adhesive, the formulations were kept as basic as possible. Different grades of naphthenic and paraffinic oils were included as plasticiser in the formulations. Additives and stabilisers were excluded in all formulations.
The performance of Nynas’ plasticisers was tested in two new metallocene-based polyolefin systems along with a selection of other typically available plasticisers. The tested mPOs were an olefin block copolymer (OBC) and a polyolefin plastomer (POP). The main difference between these two materials lies in their chemical structure. OBCs are quite similar to SBCs, having a hard segment that is highly crystalline and responsible for high-temperature properties, and a soft segment with lower crystallinity that delivers the flexibility and low-temperature properties. In contrast, POPs are random copolymers with very low crystallinity and density, which increases the flexibility. The amount of comonomers in POP determines the major properties.
The study consisted of mechanical tests and included sprayability, peel adhesion and dynamic mechanical analysis (DMA), which is a very useful tool in predicting the mechanical properties and long-term performance of adhesives. In addition, good thermal stability of the adhesive and its components is a key requirement for adhesive manufacturers. Therefore the effect of different plasticisers on thermal stability was also determined by monitoring colour, viscosity and performance change of the material during thermal ageing.
Figure 2. Peel performance of oil in polyolefin plastomer (POP) based adhesive. (TWO=technical white oil)
Compared with other plasticisers included in the tests, the overall performance of the Nynas oils Nyflex 222B and Nyflex 220 proved superior. Naphthenic plasticisers also exhibited benefits over paraffinic plasticisers. This is reflected in properties such as peel adhesion, cohesion and creep resistance. Looking at the thermal stability, it can be concluded that colour stability is not dependant on the hydrocarbon composition of the oil. Along with the plasticisers, the performance of OBC and POP was evaluated for use in adhesives for the construction of non-woven nappies.
Compared to the reference styreneisoprene-styrene (SIS) sample, both materials had very good mechanical properties which make them good contenders for replacing SIS in this application. The only disadvantage of POP is the low melting temperature, which might eliminate this polymer from selection.
Our study shows that Nynas' specialty oil will be fit for purpose as plasticiser in new mPO-based hot melt adhesives used in the construction of non-woven materials. We conclude that naphthenic oils will continue to play an important role in disposable nappies.
Project Manager, Nynas Naphthenics Technical Development and Market Support
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