CA2407880C - Process to manufacture high impact natural composites - Google Patents

Abstract

The use of natural fibers as reinforcing filler in thermoplastics is a relatively new innovation and has great potential to replace glass fiber products in building, consumer goods, furniture and automotive industry. Our published results demonstrated that laying natural fiber mat and polymer film in a definite pattern and orientation can enhance mechanical strength. In this invention, a unique process has been developed to manufacture high impact strength composite that contains loose natural fiber and/or a combination of loose natural fiber and natural fiber mat to develop green composites outstanding impact strength and other mechanical properties. Loose fibers used in this process can be obtained from agro-plant and wood. Mats, if used, can be obtained from similar sources. Polymer those are used include thermoplastic polyolefins (TPOs), acrylic and polyester films. This invention is especially useful for natural composites with natural fiber content above 40% by weight up to 93 wt% fiber. Film stacking and fiber spraying methods were used to construct sheet, laminates and thermoforming and compression molding were used to design a product. A range of impact strength was obtained which varies from 100 J/m to as high as 250 J/m with a significant increase in flexural and tensile strengths. Products having uniform density profiles yield maximum enhancement in impact properties.

Description

Patent Application no. 2,407, 880 Amendment Description Natural Fiber Thermoplastic Composite: Manufacturing process and use of product.
Specific Area of Invention This invention pertains to a simple and low cost process technique of manufacturing natural long fiber thermoplastic composite with improved mechanical properties. In this method, alternate stacking of loose natural fiber instead of woven or non-woven mat or filament-wound fiber yarn is used as main source of enforcement. Each layer of loose fiber is essentially covered on both sides with at least one film of thermoplastic matrix, where after the stack of alternate fiber layers and matrix films is pressed at elevated temperature and pressure for desired consolidation followed by in-press cooling. This invention also describes actual manufacturing of sample product and its potential applications in different specific areas like automotive, structural and furniture industry.
Background Over the last few years, ecological concern and stringent environmental legislation in North America and Europe has initiated a renewed interest in using natural materials to produce green products. Traditionally, glass fibers have been extensively used as reinforcement in thermoplastics in various applications , especially in auto sector, due to their better impact strength properties. However, glass fiber thermoplastics have several environmental disadvantages as glass fibers are obtained from non-renewable resources and a lot of energy is consumed in their production. Further, these products are non-recyclable and land filling is the main option to dispose off after their useful life span.
On the other hand, natural fibers offer an environmental friendly alternative to be used as reinforcement in both thermoplastic and thermo set composites. These fibers, like hemp, flax , kenaf, jute etc, have various advantages as being renewable, non-abrasive to process equipment and possible incineration at the end of their life cycle for energy recovery.. They are also very much safe during handling and less suspected to affect lungs during processing and use. Automotive applications represent the best opportunity for natural fibers thermoplastics due to some of distinctive advantages over glass fibers, like, low weight (35-40% less as compared to glass fiber), low price, better crash absorbance and sound insulation properties.
In literature, there are many comparisons available regarding the mechanical properties of natural and glass fiber composites. It is shown by B. van Voorn et al., Composites: Part A, Vol 32, 2001, 1271-1279, that stiffness of flax fiber thermoplastic is comparable or even better that of glass fiber counterparts whereas, flexural and tensile strength properties are more or less compatible. Similar comparisons have been reported by S.K.Garkhail et al., Applied Composite Materials, Vol. 7, 2000, 351-372 and Krishna Oksman, Applied Composite Materials, Vol. 7, 2000, 403-414.
However, the main challenge in the development of natural fiber composites is to improve the impact strength which is only 1 /4"' of glass fiber thermoplastics in most of cases as reported in earlier references. The extremely low values of impact strength has hindered so far the mass scale growth of this otherwise feasible product from entering high end markets , like auto sector, etc.
The other important issue in the use of natural fibers is the cost of raw material.
In current and prior art , the natural fibers are mostly used in the form of needle punched woven or non-woven mats which approximately doubles the cost of raw loose fiber.
Moreover, mats manufacturing effects the orientation of individual fibers to great extent and fibers tend to align in one direction, thereby, limiting the ultimate strength of composite.
This invention deals specifically with design aspects of fiber orientation ensuring maximum and uniform stress transfer from matrix to fibers in all directions and at the same time incorporates optimum stacking mechanism to facilitate a thorough flow of thermoplastic matrix into entire fiber body during heating and pressing cycle of manufacturing.
Description of Invention This invention is based on use of natural fibers in specifically loose and random orientation in the form of layers in combination with thermoplastic matrix to produce composite with improved mechanical properties , especially impact strength.
Moreover this invention demonstrates the better use of natural fibers in its more basic raw form rather to use unidirectional mats or lay-ups which give optimum properties in only one direction and also have appreciable higher costs.
Further, the present invention relates to use of optimum fiber content for improved mechanical properties, thereby, availing maximum environmental advantage by reducing the amount of thermoplastic resin in final product. The alternate stacking mechanism is another aspect discussed and explained in this invention. Both these issues have critical importance as in a polymer composite the ultimate strength of product depends on optimum quantity of reinforcing fibers and effective stress transfer from matrix to fibers.
Influence of fiber treatment on the significant improvement of strength properties of composite is also mentioned in this invention.
More specifically, this invention describes manufacturing process for natural fiber thermoplastic composite with significant improved impact strength, whereas the said product consists layers of loose natural fibers covered on each side by at least one thermoplastic matrix film in a lay-up stacking method. The alternate stack of long natural fibers and matrix films is subjected to elevated temperature and pressure for a predetermined time period followed by a cooling cycle, all done in one step.
However, pre-drying of fibers is essential to promote effective bonding with hydrophobic matrix.
The natural fibers used in this invention for reinforcement of thermoplastic composite are:
- Renewable, long bast fibers of hemp and flax.
- Recycled urban wood fibers.
- Old Newprint (ONP) fibers.
However, other plant fibers, e.g jute, kenaf, sisal, and agri-residues may also be used.
The thermoplastic matrix employed in said inventive procedure is commercial polypropylene which has lowest cost, density and water absorption among commonly used thermoplastics. Its low process temperature is also advantageous to avoid natural fiber degradation.

Fiber treatment was achieved with silane coupling agent to achieve additional improvement in mechanical properties, especially impact strength.
The process of this invention is characterized in that the natural bast fibers are used without any extra processing of needle punching, weaving or mat making.
Further, the said inventive process introduces alternate stacking of fibers and matrix as compared to prior art mentioned in WO X2/064670, where fiber layers are grouped in the middle of stack. The authors of this patent have observed in particular that matrix does not flow evenly into whole body of fibers when more than two matrix films/foils are stacked, especially at the top or bottom ends of stack. Moreover, there is also risk of matrix being melting away from edges during process.
This invention demonstrates various advantages over the prior art;
- Significantly improved shock absorbing properties, surpassing all existing natural fiber composites - Fiber raw material requires minimum processing, thereby significant potential savings in terms of material cost and energy consumption compared to using natural fiber mats or glass fibers.
- One step process to make products.
- Value added utilization of lingo-cellulose fibers from agro, wood and other recycling industry.
- Improved mechanical performance with additional step of fiber treatment.
- Specific strength properties of said composite material approaches to glass fiber products.
- Utilization of optimum natural fiber quantity in composites, thereby reducing the use of thermoplastic content which gives an added environmental advantage.
Description of drawings Figure 1: Process schematic of lay-uplstacking and compression molding.
Figure 2: Process schematic of thermoforming.

Figure 3: Effect of fiber orientation, type and frber treatment on impact properties of natural fiber thermoplastics.
Figure 4: Diagram illustrates influence of fiber content of loose,~ber on impact properties of composite.
Figure 5: Fiber content versus ,flexural and tensile strength of loose hemp fiber/PP
composite.
Examples Influence of fiber orientation, content, and percentage of coupling agent on the mechanical properties of composite in said stacking /lay-up method was demonstrated through practical experiments. Different types of fibers were used to illustrate the versatility and flexibility of the process as well. Representative samples were tested for tensile, flexural and impact values Lav-Un Cofiguration The typical film stacking method used in this invention is known per se. This inventive process is characterized by even distribution of fiber and matrix in the said lay-up as shown in Figure 1 as compared to prior art in which reinforcement fibers are placed in one central location of stack. The use of long and random loose plant fibers is another highlight of this invention. Each layer of fibers in the stack is essentially bound on each side by at least one film of thermoplastic matrix. The stack is subjected directly to high temperature and pressure without any extra step of vacuum heating. The consolidated stack is cooled under pressure to ensure dimensional stability of product.
It is shown through various experiments that a very significant increase of about 90% in impact strength is achieved by using long loose fibers instead of pre-impregnated non-woven mats. It is further demonstrated that by treating fibers with silane coupling agent, an extra 15% increase in impact strength can be achieved.
Other experiment details are mentioned as follows.
Fiber Tyues Loose Fibers:
The loose virgin fibers used in the experiments were hemp and flax. Both these kinds of fibers were used separately to make representative samples. The composite grade hemp fibers, 75mm average length, were arranged from Hempline Inc. Canada while the flax fibres were extracted from Fall-1999 crop having fiber length between 45 and 145mm.
Fiber Mats:
For comparative study, the non-woven needle punched mats (Bastmat 108) of hemp fibers were also used which were supplied from same source. The hemp fiber content in non-woven mats by weight was 80% while the rest was polypropylene. The average basis weight of these mats was 265 g/m2 + 8.
Recycle fibers:
Two different kinds of recycled fibers were also used to demonstrate the effectiveness of this inventive process. Recycled wood fibers from recovered urban timber were sourced from CanFibre of Lackawanna LLC, New York, NY. The fibers had length from 1 to mm. The second type of recycled fibers were obtained by shredding ordinary newsprint (ONP) in Wiley Mill through 2mm sieve . Newspaper strips of 1-1.5 inch wide were fed into top of Mill and flake type fibers obtained through sieve were transformed into several layers to be used in combination with matrix films. The tensile strength of original newsprint was determined according to Tappi Test Method and was found to be 27.5 MPa in machine direction(MD) and 8.3 MPa in cross direction(CD).
Thermoplastic Polymer Homopolymer grade polypropylene(PP) was supplied by Copol International Ltd.
Canada with a trade name of H 101 in form of films having thickness of 100 micron. As per supplier's data, the ultimate tensile of this general purpose polypropylene along TD is 37 MPa whereas secant modulus is 0.8GPa.
Matrix and Fiber Modification Both matrix and reinforcement fibers were treated with coupling agents to enhance compatibility and observe the effect on mechanical properties.
- Matrix treatment: 5% by weight of MA-PP (Epolene E-43, arranged from Eastman Chemical Inc) was blended with pure PP in twin screw mixer (C. W.
Brabender) at 185°C for 5 minutes. The extruded material was transformed into thin sheets to be used as matrix in lay-up process.
- Fiber treatment: Hemp loose fibers were treated with 3-aminopropyltriethoxysilane (0.5% by weight of fibers) in quick aqous method and subsequently dried to be used in said inventive process in combination with un-treated matrix.
Process Film stacking Fiber mats and loose fibers were first dried at 105 °C for two hours before arranging into lay-up.
In case of hemp mats, the mats and polypropylene films were cut into size of 200X200mm. The stacking of mats and polypropylene films was arranged in such a way that each mat had at least one matrix film on each side. Typically, nine mats were used in each lay-up with ten to eleven polypropylene films. The fiber content in final product was maintained at 60% by weight, which corresponds to 47.8% volume fraction.

All types of loose fibres were transformed separately into shape of layer/mat manually before film stacking step. In case of hemp and flax fibres, it was very convenient to get stable and uniform layers (200X200 mm) like a non-woven mat due to longer fibre length and physical entanglement among fibres. However the ONP
and wood fibres were difficult to handle because of fluffiness and shorter length.
Therefore a wooden frame was used to facilitate layer making process. Again, 60% fiber content by weight was maintained in Manufacturing of Composite Compression Molding Representative samples of composites based on hemp fiber mats and each type of other cellulose loose fibers, both virgin and recycled, were manufactured by one-step film-stacking method which is also reported by S.K.Garkhail et al., Applied Composite Materials, Vol. 7, 2000, 351-372. As already mentioned, pre-dried fibers and matrix films (200X200mm) were stacked alternately in a lay-up shape. Compression molding was achieved in a hydraulic press, V~abash, having 50-ton capacity with air/water cooling arrangement. The heating time was 8-10 minutes at temperature range of 190-200°C and compression of I 5-25 bar. The press was cooled at the end of heating/impregnation cycle to 50°C in about 4 minutes before completing the compression molding process.
Thermoformin Three dimensional prototypes were also manufactured by using thermoforming process as shown in Figure 2. Dupont Mylar (polyester) films were used as releasing agent between mold plates and combination of cellulose fibers and matrix.
Compression temperature was maintained at 205 °C for about ten minutes to complete the molding process.

Testing Izod "notched" impact testing was carried out on Tinius Olsen (92T Impact T.M) test machine according to ASTM D 256. The specimens had depth of 12.7mm and length of about 64 mm. The testing machine had inbuilt processor to calculate absorbed energy in J/m. Ten samples of each type of molded composite were tested to get average and standard deviation values.
Flexural test (3-point bending) was conducted on Zwick-z100 tester according to standard test procedure D-790 with machine speed of Smm/min and span width of 53mm.
Specimens were l2.Smm wide and 150 mm long. The tensile testing was done on same machine and specimens were prepared according to test method D-638 while the crosshead speed was maintained at 2mm/min. Five samples of each type of manufactured composites were tested for flexural and tensile testing.
Average specific gravity of some composite samples was determined by using an X-ray density profiler, QMS (QDP-01X) .Specimens had dimensions of SOXSOmm with varying thickness from 2 to 3 mm. The scanning was done through five pre-determined zones across the thickness of specimen.
Results Effect of fiber orientation A comparison among mat/PP and random long fiber/PP composite is shown in Table while keeping the same fiber content in product. The non-woven needle punched mats have usually unidirectional fibers which give anisotropic properties. The use of random loose fibers as reinforcement in polypropylene give exceptional high impact energy, almost 90% increase compare to mat/PP combination, as shown in Figure 3. The results of loose flax fiber/PP composite with same amount of fiber content is also shown in same table.
Effect of coupling agents As already mentioned , two different types of coupling agents were used separately to enhance the mechanical performance of thermoplastic.
The use of 5% Malefic-anhydride polypropylene (MA-PP) in polypropylene matrix improved the flexural strength by 15% while the tensile strength was improved by 17% as shown in Table 2.However the impact energy remained almost same.
The second compatiblizer, 3-aminopropyltriethoxysilane, was used to treat loose hemp fibers at the ratio of 0.5% of fiber weight. It improved the flexural and tensile properties by 5 and 15% respectively, whereas the impact energy is also improved by 15% compared to non-treated loose fibers.
Effect o fiber content Table 3 shows the results of fiber content influence on the mechanical properties of composites manufactured in laboratory. In this study, controlled fiber length of 40mm was used in random fashion combined with polypropylene films in alternate film stacking method.
As anticipated and discussed in literature, fiber content has profound effect on mechanical strength of composite. The trend in Figure 4 and 5 and results of Table 1 indicate an optimum fiber quantity of about 60-65% for best results. Beyond this, process-ability becomes difficult. This optimum amount of fiber is about 20%
more compare to prior art and current conventional applications of natural fiber composites where fiber content is usually maintained around 50%. The high proportion of natural fiber ultimately reduces the consumption of thermoplastic which is a high energy consuming product in itself.
Random recycled fibers The value added utilization of recycled fibers, urban timber and old newspaper, in thermoplastics was achieved in similar way as with virgin fibers. Fiber content was maintained at 64% and results of testing are shown in Table 4. This study shows the technical feasibility to use these fibers in alternate stacking lay-up method.
The lower strength properties of these products may be attributed to inherent low strength of wood fibers and multiple recycling effect, especially in case of newsprint.
However, these type of products may find use in decorative paneling or other non-structural applications.
Conclusions - The impact properties of random loose natural fibers and PP composites in alternating stacking inventive technique are far superior than matlPP
combinations.
- Fiber treatment with silane can further enhance the strength properties of final product in same way of manufacturing.
- MA-PP treatment does not improve impact energy, however tensile and flexural properties are appreciably improved.
- Overall mechanical performance of natural fiber composites manufactured through this inventive procedure largely depends on fiber content. The optimum fiber content for best results is about 60-65%.
- Recycled fibers can be incorporated as reinforcement media in thermoplastics in the said inventive procedure.
Potential Applications This inventive process will eliminate or significantly replace non-woven mats of natural fibers and glass fiber mat-based composites in automotive, household and upholstery products. The auto industry seems to benefit more from this new technology as the improved specific mechanical properties and low density of natural fiber thermoplastics can play a dominant role in replacing conventional glass fiber products in most of applications.
European auto industry already uses significant amounts of plant fibers in high-end brand names of car models and by end of 2005, new environmental legislation requires to produce 95% recyclable automobiles. According to latest report by Principia Consulting, Global Hemp Newsletter, March 2003,the current consumption of all kinds of natural fibers in North America and Western Europe combined is about 0.67million metric tons. However, auto industry is Western Europe consumes about half of its total natural fiber composites and this consumption is expected to g row steadily due to obvious advantages in using these fibers and meeting new legislation. In North America, only 8-10% of its total natural fiber composite demand goes to auto sector while rest is used up in decking and building industry. The technology adopted in this invention to produce high performance molded composites can play a pivotal role in utilizing the true potential of ligno-cellulose fibers in household furniture, upholstery and especially auto industry.
Claims 1. A manufacturing process to produce thermoplastic composite in which reinforcement fibers in the form of loose bundle layers are stacked alternately with thermoplastic matrix, whereby each layer of loose fiber is covered by at least one film or foil of matrix material on each side, thereafter hot pressing the alternate film stack/lay-up at high temperature followed by cooling under same consolidation pressure to manufacture desired composite material, characterized in that the loose, random, and un-processed fibers are used and distributed evenly in the said film stacking arrangement where each layer of fiber and matrix material has equal pre-defined quantity by weight.
2. A process according to claim 1, characterized in that fiber mats can be used in similar lay-up method.
3. A process according to claim 1, characterized in that interfacial bonding between cellulose fibers and matrix is introduced by using pre-treated matrix films with malefic-anhydride polymers of polypropylene (MA-PP) or polyethylene (MA-PE).

Claims (12)

1. A manufacturing process to produce thermoplastic composite in which reinforcement fibers in the form of loose bundle layers are stacked alternately with thermoplastic matrix, whereby each layer of loose fiber is covered by at least one film or foil of matrix material on each side, thereafter hot pressing the alternate film stack/lay-up at high temperature followed by cooling under same consolidation pressure to manufacture desired composite material, characterized in that the loose, random, and un-processed fibers are used and distributed evenly in the said film stacking arrangement where each layer of fiber and matrix material has equal pre-defined quantity by weight.

2. A process according to claim 1, characterized in that fiber mats can be used in similar lay-up method.

3. A process according to claim 1, characterized in that interfacial bonding between cellulose fibers and matrix is introduced by using pre-treated matrix films with maleic-anhydride polymers of polypropylene (MA-PP) or polyethylene (MA-PE).

4. A process according to claim 1 and 2, characterized in that reinforcement fibers fall in one of following categories:
a. Cellulose bast fibers obtained from plants of hemp, flax, kenaf, sisal and jute.
b. Agri-residue fibers, such as rice husk and wheat straw.
c. Virgin or recycled wood fibers.

5. A process according to claim 1 to 3, characterized in that matrix selected is from thermoplastic family of polymers consisting either of homo polypropylene (PP) or co-polymers of polypropylene and polyethylene.

6. The thermoplastic polymer manufactured through the said inventive process consists essentially the following:
1. Alternate stacked layers of loose ligno-cellulose fibers and/or mats in combination with thermoplastic matrix films distributed evenly in stacked lay-up.
2. The matrix films may or may not be treated with coupling agent.

7. The manufacturing process of compression molding consists following steps:
1. Film stacking of fiber layers and matrix films in alternate arrangement in such a way that each layer of natural fibers is covered on both sides by at least one film of matrix.
2. Hot pressing of film stack at elevated temperature and pressure followed by cooling under same consolidation to manufacture desired thermoplastic composite.

8. Thermoforming process can be also used to make three dimensional/intricate products by using same raw materials as mentioned in claim 4 and 5.

9. In the product of claim 6, the reinforcement material belongs to any of plant fibers, agri-residuals or virgin/recycled wood fibers as mentioned in claim 4.

10. The matrix in the product of claim 6 is essentially of thermoplastic family as mentioned in claim 5.

11. The coupling agents in the product of claim 6 can be either impregnated into thermoplastic films or cellulose fibers as mentioned below:
a. The matrix is treated with maleic-anhydrie co-polymers of polypropylene or polyethylene.
b. Reinforcement fibers can be treated with Silane solutions through quick aqous method if required.

12. The commercial use of said product according to any claims 6 to 10 in auto sector, general purpose household, upholstery and semi-structural construction applications as mentioned in potential application section.