Introduction
Synthetic polymeric fibers are increasingly gaining usage, both at homes and in the industry. This raises concerns, as polymers such as polypropylene, polyethylene, terephthalate (PET), and nylon used to produce fiber are non-biodegradable and, therefore, not environmentally friendly. Many synthetic polymeric fibers dumped in the environment make landfills limited and emission of incinerators promotes global warming.
The need for environmental conservation led to the development of environmentally friendly polymers which are biodegradable. This, according to Mooney et al. (1), enables products developed from such polymers degrade without interfering with the environment by breaking down into simpler components by macromolecular chain scission. Products made from nonwoven fibers significantly promise to conserve the environment, as they are biodegradable.
There are various biodegradable polymers available today. Bhat, Gulgunje, & Desai (2) gave examples of both natural and synthetic polymers to include proteins, polyesters, polyhydroxybutyrate and polysaccharides. An example of a polyester polymer that is promising to be friendly to the environment is Poly (Lactic Acid) (PLA), since it is biodegradabile. This is partly because it has decent mechanical properties including the ability to be processed thermoplastically and being able to biodegrade.
In addition, it is durable and crisatllizes within a short duration. Bhat, Gulgunje, & Desai (2) add that when the L-form, Meso form, and D-form of the PLA fiber are properly controlled, they exhibit many properties. Mechanical properties are a result of methods applied when bonding. This paper discusses PLA fibers and various methods used in fiber bonding, specifically PLA fibers.
PLA Fibers
It is crucial to discuss how PLA fibers are manufactured before looking at methods used for its bonding. Unlike other polymers whose monomers and energy are used for their manufacturing are got from oil and gas reserves, which decline; PLA is manufactured using monomers got from crops - a renewable resource. Farrington, Lunt, Davies, & Blackburn (3) explain that through the process of photosynthesis, plants produce starch as one of the byproducts. The starch is then extracted from plants and converted to glucose through hydrolysis. Other byproducts, such as carbon dioxide, are fermented to lactic acid.
Most PLA fibers produced today use glucose from corn. This is because only little amount of corn glucose is required to produce the considerable amount of PLA fiber. According to Gruber and O’Brien (4), about less than 0.5% of the corn produced by the United States yearly helps in the manufacturing of 500,000 tons of PLA.
PLA is produced either by directly condensing lactic acid or through ring opening process. Condensation of lactic acid removes water and uses solvent in a vacuum under the high temperature. This process only removes limited amount of water and is suitable in producing polymers with low or average molecular weights.
Properties of PLA Fiber
PLA fibers share many properties with other fibers including smooth surface, crimp control and slow regain of moisture. It, however, differs with others, since it is the only fiber that can be melt-processed using natural resources which can be renewed yearly. Physically, Farrington et al. (3) points that PLA fibers have circular cross-sections with smooth surfaces and density of 1.25g/cm below that of natural fibers and PET.
The fiber’s refractive index of 1.35 to 1.45 make it suitable for making products of different shapes and still provide antisoiling features. It has the glass transition temperature (Tg) between 55-650, melting temperature (Tm) between 160-1700 for L- and D-polymers.
Fiber Bonding
The mechanical strength exhibited by fibers is as a result of bonding. Fiber bonding is done using various methods including thermal calendar bonding, ultrasonic pattern bonding, needle bonding, melt-blown bonding and hydroentanglement bonding.
Thermal Calendar Bonding
Bhat, Jangala, & Spruiell (5) explain that thermal calendar bonding is widely used to bond nonwoven fibers because it does not require a lot of money. The process entails passing of a pre-consolidated web between two rolls which touch each other at a specified pressure and are heated up to a predetermined temperature. Bhat & Malkan (6) add that one of rolls usually has some engraved pattern on the surface to enable bonding between fibers to take place at the point where the smooth roll and the engraved pattern intersect.