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I PVC plasticizer adds value to waterbeds

Danish company AkvaWaterbeds ApS has the replaced DEI-IP plasticized PVC film in its water mattresses with PVC plasticized with phenol alkyl sulphonate. Plasticizer manufacturer Bayer Chemicals claims that the switch has provided a number of important performance benefits. Plastics Additives & Compounding reports.

Once a rare item of furniture, waterbeds have now become a means of combating sleep and back problems to be taken seri- ously. The virtual weightlessness of a body on a water-filled mattress means that pres- sure points and interruptions to circulation that may occur during the night are elimi- nated. In normal circumstances on a con- ventional mattress, these would cause the sleeper to change position up to 80 times per night -interrupting deep sleep without the person being aware of it. The evenly warm temperature of the water filling also helps counteract muscle tension. In addi- tion, since a waterbed tailored to the body gives optimum relief to the spinal column, the intervertebral disks have an excellent chance to recover overnight.

Physiotherapists often recommend waterbeds to people suffering from back problems. Those with sleep problems have also reported significant improvements in their condition. Being free from dust mites, waterbeds are also claimed to benefit those who suffer from allergies. Indeed, waterbeds are actu- ally a more hygienic alternative to foam mattresses since they are easier to clean and are not permeated by sweat and rubbed offparticles of skin. Bayer believes that the high level of acceptance that waterbeds have found even among more conservative consumers has something to do with the constant improvement in the properties of PVC film - the crucial component of a

waterbed mattress. This membrane has stringent requirements to fulfill. For example, it needs to be sufficiently soft that potential creases in the surface of the mattress do not become pressure points and its seams need to be welded very securely so that no leaks occur over the years. In addition, there are high stan- dards to be met in terms of tear strength and the elongation properties of the film - over the course of many years. Even though waterbeds are not filled under pressure and the contents will at most seep out in the event of a leak, the materi- al cannot be allowed to become prema- turely brittle. For this reason, the plasticizer in waterbed films in particular plays an important role,

Plastics Additives & Compounding March~April 2003

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C a s e s t u d y - p l a s t i c i z e r s

explains Bayer. In Akva waterbeds, phenol alkyl sulphonate plasticizers - Mesamoll ® - have proved particularly effective, says the company. These additives are claimed to have two main advantages. Bayer says that they have significantly greater saponification resistance than phthalate plasticizers and display a much- reduced tendency to migration (see Figure 1). Both of these are particularly impor- tant for a film that is exposed to a warm, aqueous medium throughout its whole life. The loss of material caused by hydrolysis - washing out or evaporation that can occur with plasticizers - may cause waterbed films to shrink by up to 10% over time. This causes the brittleness of the membrane to increase and its elastici- ty and elongation properties to decrease, says the company. Bayer says that Mesamoll has demonstrat- ed its resistance to hydrolysis in harsh weathering tests. In 5-year Florida and open-air weathering tests, PVC films plas- ticized with DEHP showed a clear drop in tensile strength and elongation at break after a few years, which can be attributed to the leaching out of the plasticizer, says the company. However, the properties of Mesamoll-based films are claimed to remain at a constantly high level for a long period of time. For example, the company adds that the tensile strength and elongation at break of PVC films plasticized with D E H P fell significantly after just two years' open-air weathering in the North Sea climate. After five years they are reduced to about two thirds of their original value, while with Mesamoll-based films Bayer claims that they remain almost at the original level (see Figure 2). After five years in a Florida climate, test samples with D E H P only achieve 47% of their original value for elongation at break, while those with Mesamoll still have 83% of their starting level. According to Bayer, these findings also correlate with microscope images made of the new and weathered PVC films. While the samples plasticized with phenol alkyl sulphonic acid esters retained a smooth surface for several years, films consisting of DEHP soon showed a fissured structure

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Figure 3:The reason for the gradual deterioration in the material properties and the formation of hairline fractures in PVC films in conditions promoting saponification lies in the degradation and release of plasticizers. On the right is the fissured surface of a test piece containing DEHP after two years of weathering, while on the left is a test piece plasticized with Mesamoll after more than double that length of time - magnified 40 times. (Source: Bayer)

with 'islands', in which only PVC and solid components of the formulation can be detected, when observed under the microscope (see Figure 3). This was due to loss of plasticizer, claims the company. Phenol alkyl sulphonic acid esters also offer saponification resistance, which ensures that the waterbed filling does not become a habitat for microorganisms. Bayer says that Mesamoll provides no basis for life for the bacteria and fungi that can feed on plasticizers such as DEHP. In addition, this plasticizer is not attacked by the more aggressive cleaning materials and that even frequent cleaning (which is desirable for hygiene reasons) does not cause a deterioration in the PVC surface properties. Bayer believes that the balance between tear strength and plasticity can also be maintained with Mesamoll. According to Akva, PVC plasticized with Mesamoll has improved tear strength and although it is 10% stronger it is more flexible than other PVC grades. This means increased comfort when lying on the waterbed, claims the manufacturer. It adds that the number of complaints received since the new plasticizer was

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introduced has dropped by about two- thirds. At Akva, the durability of the film material is tested in a combined ageing and impact test, and according to the processor the material withstands 400% more impacts than materials plasticized using phthalates. Finally, Bayer claims that Mesamoll has a low migration tendency and complies with the DIN EN 71-5 standard for toys. In production, Bayer says that since Mesamoll gels faster and at lower temper- atures than most other plasticizers, pro- cessing time is cut significantly and the amount of energy required in production is reduced. Akva is exploiting this to produce particu- larly soft seams. The high frequency weld- ing process enables almost transition-free seams, which together with a new cast corner, offers a high quality water mat- tresses.

Contact: Bayer Chemicals Tek +49 2133 5i 5441 E-maik eberhard.kuckert.ek@ bayerchemicals, corn www.polymer-additives.de

Plastics Additives & Compounding March~April 2003