Patent Application
20070269644
A method of making a porous, fiber reinforced thermoplastic sheet having increased lofting properties is described below in detail. The method includes generating a z-axis orientation of a portion of the reinforcing fibers, at least about 5 percent by weight of the reinforcing fibers. The x-y plane encompasses the width and length of the sheet, while the z-axis direction encompasses the thickness of the sheet. Increasing the number of reinforcing fibers in the z-axis direction permits added loft, or expansion of the sheet, during forming to a greater thickness than known thermoplastic sheets with the same reinforcing fiber loading.
Referring to the drawing,
Core 12 is formed from a web made up of open cell structures formed by random crossing over of reinforcing fibers 20 held together, at least in part, by one or more thermoplastic resins 22, where the void content of porous core 12 ranges in general between about 5% and about 95% and in particular between about 30% and about 80% of the total volume of core 12. In another embodiment, porous core 12 is made up of open cell structures formed by random crossing over of reinforcing fibers 20 held together, at least in part, by one or more thermoplastic resins 22, where about 40% to about 100% of the cell structure are open and allow the flow of air and gases through. Core 12 has a density in one embodiment of about 0.1 gm/cc to about 1.8 gm/cc and in another embodiment about 0.3 gm/cc to about 1.0 gm/cc. Core 12 is formed using known manufacturing process, for example, a wet laid process, an air laid process, a dry blend process, a carding and needle process, and other known process that are employed for making non-woven products. Combinations of such manufacturing processes are also useful.
Core 12 includes about 20% to about 80% by weight of reinforcing fibers 20 having an average length of between about 5 mm and about 75 mm, and about 20% to about 80% by weight of a wholly or substantially unconsolidated fibrous or particulate thermoplastic materials, where the weight percentages are based on the total weight of core 12 In another embodiment, core 12 includes about 30% to about 55% by weight of reinforcing fibers 20. In another embodiment, core 12 includes reinforcing fibers 20 having an average length of between about 5 mm to about 50 mm, and in another embodiment between about 5 mm and about 25 mm. Suitable reinforcing fibers include, but are not limited to metal fibers, metalized inorganic fibers, metalized synthetic fibers, glass fibers, graphite fibers, carbon fibers, ceramic fibers, basalt fibers, inorganic fibers, aramid fibers, and mixtures thereof. Also, natural reinforcing fibers can be used, for example, kenaf fibers, jute fibers, flax fibers, hemp fibers, cellulosic fibers, sisal fibers, coir fibers, and mixtures thereof.
In the exemplary embodiment, reinforcing fibers 20 having an average length of about 5 mm to about 75 mm are added with thermoplastic powder particles, for example polypropylene powder, to an agitated aqueous foam which can contain a surfactant. The components are agitated for a sufficient time to form a dispersed mixture of the reinforcing fibers 20 and thermoplastic powder in the aqueous foam. The dispersed mixture is then laid down on any suitable support structure, for example, a wire mesh and then the water is evacuated through the wire mesh forming a web. To increase the lofting capability of core 12, a portion of reinforcing fibers 20 are orientated into a z-axis direction. What is meant by z-axis direction is that a reinforcing fiber 20 is orientated outside of the x-y-plane of core 12. The x-y plane encompasses the width and length of core 12, while the z-axis direction encompasses the thickness of core 12. The remainder of reinforcing fibers 20 are typically substantially parallel to the x-y plane of core 12.
In one embodiment, mechanical action is used to orientate a portion of the reinforcing fibers in the z-axis direction. Any suitable mechanical action can be used, for example, tines oscillating back and forth parallel to the z-axis of core 12.
In another embodiment, a hydroentangler having a plurality of water jets in a predetermined pattern directed at the web is used to orientate the reinforcing fibers in the z-axis direction. With an increased number of reinforcing fibers in the z-axis direction rather than an x-axis direction or a y-axis direction, core 12 can loft, or expand, to a desired predetermined thickness. Any excess water from the hydro-entangling process is evacuated from the web.
In another embodiment, a high consistency headbox is used to form the web which orientates a portion of the reinforcing fibers in the z-axis direction. High consistency headboxes are typically used in papermaking processes in conjunction with high consistency paper making stock. It has been found that high consistency headboxes that produce microturbulence in the dispersed mixture of the reinforcing fibers and thermoplastic powder in the aqueous foam can be used to generate z-axis orientation of a portion the reinforcing fibers. High consistency headboxes are commercially available from Metso Corporation, Helsinki, Finland.
The amount of z-axis orientated fibers depend on the composition of the composite, the desired loft, and the application of the composite. In one embodiment, the amount of z-axis direction fibers is at least about 5%, in another embodiment, about 20% to about 50%, and in another embodiment at least 50%, the percentages being weight percent.
The web is dried and heated above the softening temperature of the thermoplastic powder. The web is then cooled and pressed to a predetermined thickness to produce a composite sheet having a void content of between about 5 percent to about 95 percent.
The web is heated above the softening temperature of the thermoplastic resins 22 in core 12 to substantially soften the plastic materials and is passed through one or more consolidation devices, for example calendaring rolls, double belt laminators, indexing presses, multiple daylight presses, autoclaves, and other such devices used for lamination and consolidation of sheets and fabrics so that the plastic material can flow and wet out the fibers. The gap between the consolidating elements in the consolidation devices are set to a dimension less than that of the unconsolidated web and greater than that of the web if it were to be fully consolidated, thus allowing the web to expand and remain substantially permeable after passing through the rollers. In one embodiment, the gap is set to a dimension about 5% to about 10% greater than that of the web if it were to be fully consolidated. A fully consolidated web means a web that is fully compressed and substantially void free. A fully consolidated web would have less than 5% void content and have negligible open cell structure. In alternate embodiments, the web can be manufactured in unconsolidated rolls or unconsolidated slitted sheets. The unconsolidated web or sheets are then reheated and consolidated at a latter time and/or location using conventional IR dryers, conventional dryers, calendar rolls, batch press, double laminators, and the like.
Particulate plastic materials can include short plastics fibers which can be included to enhance the cohesion of the web structure during manufacture. Bonding is affected by utilizing the thermal characteristics of the plastic materials within the web structure. The web structure is heated sufficiently to cause the thermoplastic component to fuse at its surfaces to adjacent particles and fibers.
In one embodiment, individual reinforcing fibers 20 should not on the average be shorter than about 5 millimeters, because shorter fibers do not generally provide adequate reinforcement in the ultimate molded article. Also, fibers should not on average be longer than about 50 millimeters since such fibers are difficult to handle in the manufacturing process.
In one embodiment, in order to confer structural strength, reinforcing fibers 20 have an average diameter between about 7 and about 22 microns. Fibers of diameter less than about 7 microns can easily become airborne and can cause environmental health and safety issues. Fibers of diameter greater than about 22 microns are difficult to handle in manufacturing processes and do not efficiently reinforce the plastics matrix after molding.
In one embodiment, the thermoplastics material 22 is, at least in part, in a particulate form. Suitable thermoplastics include, but are not limited to, polyolefins, including polymethylene, polyethylene, and polypropylene, polystyrene, acrylonitrylstyrene, butadiene, polyesters, including polyethyleneterephthalate, polybutyleneterephthalate, and polypropyleneterephthalate, polybutyleneterachlorate, and polyvinyl chloride, both plasticized and unplasticized, acrylics, including polymethyl methacrylate, and blends of these materials with each other or other polymeric materials. Other suitable thermoplastics include, but are not limited to, polyarylene ethers, acrylonitrile-butylacrylate-styrene polymers, amorphous nylon, as well as alloys and blends of these materials with each other or other polymeric materials. It is anticipated that any thermoplastics resin can be used which is not chemically attacked by water and which can be sufficiently softened by heat to permit fusing and/or molding without being chemically or thermally decomposed.
The thermoplastic particles need not be excessively fine, but particles coarser than about 1.5 millimeters are unsatisfactory in that they do not flow sufficiently during the molding process to produce a homogenous structure. The use of larger particles can result in a reduction in the flexural modulus of the material when consolidated.
While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.
