TEXTILES.ORG
By H. Toyoda
Abstract
Both polytetrafluoroethylene(PTFE)-and polyvinyl chloride(PVC)-coated fabrics are employed today in different
type of tents and architectural membrane structures all over the world. However, there is no easy and effective
recycling method for these fabrics because of the difficulty of separation of the fibers from the coating resins. From the
environmental viewpoint, Taiyo Kogyo has succeeded in developing Kenafine™ that is a green fabric for architectural membrane structures made by weaving kenaf bast fiber with polyester fiber as a base fabric. Kenafine™ can be easily
recycled to produce paper products.
The fabric is a multi-functional coated one which has recyclable, self-cleaning effect and an antibacterial activity.
Introduction
Kenaf is a species of Hibiscus and an annual or biennial herbaceous plant. It grows faster and absorbs more carbon
dioxide for a fixed period of time, compared with other trees and plants. Thus, it attracts much attention as a material for
environmental preservation and, then, instead of wood it has been used as a raw material for production of papers [1],
interiors of cars [2, 3], electronic products [4] and bio-fibers [5].
PTFE-and PVC-coated fabrics show a lot of excellent properties, such as high tensile strength, long-durability,
flame-retardancy, flexibility and so on.
The lifetimes of these common coated fabrics are estimated to be over 20 years and ultimately the plastic wastes are
disposed by landfill. When the processing by incineration is carried out for these materials, harmful gases may be
evolved and/or they exert a bad influence on global atmosphere by generating carbon dioxide, i.e., global warming.
Our basic principle of design is to make an easily recyclable fabric by using environmentally friendly materials such
as bio and /or natural material. Thus, we tried to apply the kenaf bast fiber as a base fabric and finally we succeeded in
producing the fabric for architectural membrane structures: Kenafine™. The fabric is recyclable, i.e.; it can be recycled
into paper products after use. Since natural fiber is used in Kenafine™, it is much closer to carbon-neutral compared to
the fabric composed of 100% polyester fiber.
It seems that the utilization of this fabric contributes to suppression of global warming.
Two functions were also added to Kenafine™, that is, a self-cleaning effect by titanium dioxide (TiO2) photocatalyst on
the outer surface and the silver-antibacterial function on the back surface.
Development Stage
When we use a new fabric for architectural membrane structures in Japan, we have to obtain Membrane Approval
from Ministry of Land, Infrastructure and Transport.
Table-1 shows some of examples for the necessary minimum requirement items for the authorization [6].
Base fabric
Firstly, the original kenaf bast yarn of 3.5 s/1(jute yarn number) was obtained and then a base plain weave fabric was woven. Tensile strength of a base fabric was measured, however, our target strength (2000N/3cm) was not satisfied: the measurement value was less than 1000N/3cm.
We then combined polyester yarn and kenaf yarn together in the weave in order to reinforce tensile strength. Another important point is the recyclability. The base fabric which contains kenaf yarn has much cellulose and lignin as the constituent. We surmised that this might be possible to recycle Kenafine™ into paper products.
We used a special pulverizer to investigate whether it would recycle to pulp. As a result, it was found that pulp- or cotton-like flakes can be produced if the weight of kenaf fiber in the base fabric is more than 15%.
Coating material
At the first stage, non-halogen resins were chosen among the coating material, such as PU, PE, PP, and EVA. However, these resins did not satisfy from the aspect of cost-performance, such as durability, flame retardancy, dirt-repellency, and so on.
As a result, PVC was employed for the coating material because this showed excellent mechanical and surface properties, and also satisfied adequate cost. As the PVC coating contains a plasticizer of 30-50wt% , it becomes especially liable to dirt-adhesion. In order to resolve this issue, we treated the coating with titanium dioxide (TiO2) which allowed photocatalyst performance. For application of the photocatalyst on the surface, we used a protective-adhesive layer between the PVC layer and TiO2 layer (4). Anti-bacterial agent of silver was treated to the back side surface to keep the inside of the membrane structures clean. Fig.1 shows the structure of Kenafine™.
Results and discussion
Properties as a coated fabric
We tested Kenafine™ on the important items for actual usages for the architectural membrane structure. Table-1 shows the test results. All properties such as mechanical, flame retardancy, photocatalytic, and antibacterial were satisfied. Therefore, Kenafine™ can be use for the architectural membrane structure.
Recycling
In the case of recycling of typical PVC-coated polyester fabrics, the PVC and the fiber have to be separated where
the solvent and heat treatment are needed [7, 8]. The concept of our recycling system is simple and also environmental
friendly. In the whole process, we could recycle Kenafine™ by using only water as a dispsersant without separating
coating material from the fiber.
A sheet of Kenafine™ was cut approximately 1-2cm square (Fig. 2) and then put into a special pulverizer (Fig. 3).
The pulverizer is a kind of grinder and is able to grain the samples only with water. After the pulverizing processing,
pulp- or cotton-like flakes were obtained (Fig. 4). The flakes contain kenaf, polyester and PVC.
In the next step, a manual paper-milling test was carried out by using the obtained flakes with water as a dispersant, thus,
we were able to succeed in paper making (Fig. 5).
It was proved that Kenafine™ was easily recyclable to papers by using only the special pulverizer with water as a
dispersant.
Summary
We succeeded in developing an environmentally friendly coated fabric Kenafine™ by using a biomass material,
kenaf. The fabric is recyclable and the recycling way is to produce paper products from the material directly without any
use of solvent and heat-treatment.
We could demonstrate one example for adaptation to the recycling society by this development. Even if large scale
of architectural membrane structures by using Kenafine™ will be employed, these will become valuable resources, not
wastes after the lifetime.
Technical Research Center, Taiyo Kogyo Corporation, Osaka, Japan. Email: th000902@mb.taiyokogyo.co.jp
References
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