CA2477564C - Process to manufacture greener thermosetting composites of pre-shaped structure - Google Patents

Abstract

A process to prepare structural composites for automotive, aerospace, furniture and sports goods applications whereby composites are manufactured by a modified resin transfer molding process with resins in liquid form either modified or unmodified were injected under pressure into a cavity of a pre-shaped design containing fibres in nonwoven, loose or mat form whereby fibres obtained mainly from natural origin but in few cases they are combined with synthetic fibres to a lesser extent are layered in the pre-shaped mold in single or multiple layers whereby the fibres. The molded products exhibit excellent mechanical strength and durability suitable for applications in automotive exterior, interior parts, aerospace parts, furniture and sports goods applications.

Description

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DESCRIPTION
Technical Field of the Invention This invention pertains to manufacturing of natural fibre reinforced thermoset composite materials through a modified resin transfer molding technique. In this method, moisture sensitive reinforcement fibres can be used in a number of different arrangements to impart desired properties to finished product.
Background of the Invention Natural and wood fibre composites are manufactured by combining wood or other natural fibres such as flax, hemp, jute or kenaf, with polymers including polyethylene, polypropylene, or polyvinyl chloride (PVC). Composites based on natural and wood fibres are one of the fastest growing markets in the plastics industry. They can be used to produce products for building, automotive, infrastructure and consumer applications.
These types of composites present many advantages compared to synthetic fibre reinforced plastics such as low tool wear, low density, cheap cost, availability and biodegradability. For high performance composites bast fibres, extracted from the stems of plants such as jute, kenaf, flax, and hemp, are widely accepted as the best candidates due to their very good mechanical properties. Hemp especially was shown to have very promising tensile properties for such applications 1-3.
Natural fibres consist mainly of cellulose fibres. These fibres are made of microfibrils in a matrix of lignin (or pectin) and hemicellulose. The strength and stiffness of the fibres are provided by hydrogen bonds and other linkages. The overall properties of the fibres depend on the individual properties of each of its components. Hemicellulose is responsible for the biodegradation, moisture absorption and thermal degradation of the fibre. On the other hand lignin (or pectin) is thermally stable but is responsible for the UV degradation of the fibre. On average natural fibres contain 60-80%
cellulose, 5-20%
lignin (or pectin) and up to 20% moisture'.
The thermal stability of the reinforcing fibres is a key parameter in composite processing, especially in the case of thermosetting resins and their exothermic curing behaviour.

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Wielage et al:4 studied the thermal stability of flax and hemp fibres using differential scanning calorimetric (DSC) and thermo-gravimetric (TGA) methods. Their results suggest that hemp and flax fibres have the thermal stability to endure thermoset cure reactions encountered during composite manufacturing.
Another important aspect is the moisture content of natural fibres. These fibres are hydrophilic and absorb water. The moisture content can go as high as 20%, but in most cases it will be in the range 5 to 10%. This can affect the final properties of the composites. During processing the presence of water can create voids in the matrix and also lead to a poor adhesion of the fibres with the hydrophobic resini'3. The hydrophilic nature of natural fibres can be a problem in the finished composites as well.
Li et al.5 reviewed many papers concerning the mechanical properties of natural fibres. It was shown that the tensile properties of these fibres are not uniform along their length.
In their extensive report on "Composites Reinforced with cellulose based fibres" Bledzki and Gassan2 gave some data for various natural fibres as well. As observed previously the characteristic values of natural fibres are comparable to those of glass fibres. The strength of natural fibres greatly depends on the process used to produce them. In theory the elastic moduli of cellulose chains can reach values of 250 GPa. However there is no existing procedure to separate these chains from the microfibrils and therefore obtain such values. Right now the pulp and paper industry is able to produce cellulose fibres with moduli around 70 GPa. Moreover some experimental data obtained from flax and pineapple fibres show that the tensile strength of these fibres is significantly more dependent on the length of the fibre than for the case of glass fibres.
Natural fibres seem to be less homogeneous than synthetic fibres. From these papers it can be concluded that even if natural fibres are well suited to replace glass fibres in composite materials many improvements can still be done concerning their mechanical properties.
Experimental data giving the tensile strength, flexural strength, modulus, impact force and compressive force are available in the literature for different types of natural-fibre composites.
Research on hemp fibre composite is still in its early stage and only few publications can be found in the literature. Keller6 worked on a biodegradable system based on thermoplastic resins. The mechanical properties of the resulting composites were found to be quite low compared to polypropylene. Pervaiz and Sain7 studied the strength data for sheet molded polyolefin hemp fiber composites, and found that the tensile and impact strength of these materials were shown to be substantially lower than their glass fiber counterparts.
In this work a thermoset resin is the system of choice and hence the following paragraphs contain review of papers dealing with such polymers. Among the techniques available for the production of thermo-set composites, Resin transfer Molding is a very popular process in the automotive and aerospace industries to produce large and complex parts.
Sebe et al.8 manufactured hemp fibre/polyester composites using RTM. They obtained good quality parts with high flexural properties, but the impact strength of these materials was found to be very low. Richardson and Zhang9 presented an experimental study of the mold filling process for a non woven hemp/phenolic resin system. Fiber washing was shown to be a problem at low fibre concentration due to poor clamping. Edge flow was observed during the mold filling as well. The use of performs larger than the mold solved this problem. The injection pressure and the fiber concentration were shown to be the critical parameters to achieve proper mold filling. A few other publications presented natural fibre composites manufactured by RTM'""

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Recently hemp fibre/unsaturated polyester composites were manufactured in our lab using a Resin Transfer Molding (RTM) process12. These materials have promising mechanical properties. Surface modifications of the fibres were proposed in order to improve these mechanical properties as well as the fibre/matrix interface interaction".
The results did not provide substantial changes in the materials properties.
The strength tests gave promising results. It is anticipated that moisture in fibres is a key factor to influence the curing mechanism of resin and hence the final properties of the composites.

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Summary of the Invention In this work natural fibre reinforced epoxy vinyl ester resin composites were manufactured using a Resin Transfer Molding (RTM) process. RTM composites with fibre contents, up to 40 % by volume, were manufactured. The wetting of the fibres was very good. The resin injection time was observed to increase dramatically at high fibre contents due to the low permeability of the mat. Surface treatment of fibres provided improved moisture resistant properties and also enhanced composite properties.
Loose fibre, mats and woven as well as non-woven natural fibres were used with different design and construction. Examples of woven fabrics are jute, cotton and glass.
Examples of nonwovens are hemp, flax, hemp-polyester, and flax-polyester. The typical loose fibres were hemp, flax, soy, wheat, cotton, corn. Natural fibres were used either alone in combination with a thermosetting resin or a hybrid system where glass fibre and /or glass mat was used in addition to the natural fibres substrates. Natural fibres and their substrates used were either untreated or surface treated with any functional chemicals.
Typical examples of functional treatment chemicals were maleic coplymers, maleimide coplymer, aryl- maleimide, their quaternary salts with variable charge density and aryl or maleic content. Surface chemicals were also used with variable molecular weight. Other surface chemicals used were alkyl imines, and their combinations with rosin ester and rosin derivatives. Keeping a constant mold temperature is the key to obtain fast and homogenous curing of the part. The experimental procedure designed in this research resulted in the production of parts with a good finish and very promising mechanical properties. The performance of these samples was evaluated by measuring tensile strength and flexural strength.

Brief Description of the Drawings Figure 1: Diagram of resin transfer molding equipment Figure 2: Tensile strength of Silane treated composites (20 vol% fibres) Figure 3: Tensile modulus of Silane treated composites (20 vol% fibres) Figure 4: Flexural strength of Silane treated composites (20 vol% fibres) Figure 5: Flexural modulus of Silane treated composites (20 vol% fibres) Table 1: Summary of tensile, impact and flexural properties of composites containing 20 to 30% hemp fibre.
Detailed Description of the Invention Experiments A number of polyester/natural fibre composites were manufactured in the lab.
The final dimensions of the parts were 380mm by 380 mm by 3.4 mm thick. The mold (30), comprising two plates and made of aluminium, was opened and closed manually with 16 screws distributed around the cavity which contained fibres of choice. The two inlet ports (20) were situated under the mold and a vent port (40) was located on the top.
It was kept at constant temperature during the curing reaction by water flowing inside its upper and lower sections (50). The water, circulated in a closed loop through a tank, was kept at constant temperature with a temperature controller (60) connected to a thermocouple and an immersion heater of 2000 watts. To compensate for the heat produced during the exothermic crosslinking reaction cold water was kept running permanently in a copper coil placed in the tank. This system balanced itself around the preset temperature during the experiment. It should be noted that the thickness of the composite was defined by a frame placed between the upper and lower plates of the mold; it could therefore be modified in further experiments.
Testing The tensile and flexural strength of the composites were determined using a SATEC 110000 Materials Testing System. The tensile properties of the materials were measured following the ASTM standard method D638-99. The flexural properties were obtained according to the ASTM standard method D790-99. The size and shape of the different samples were chosen depending of their thickness as mentioned in these methods.
Example 1:
Prior to a typical experiment the surfaces of the mold were cleaned with the Frekote PM mold cleaner and then coated with the Frekote B-15 Sealer and the FrekotelD 700-NC mold release agent. Once these coatings were cured layers of natural fibres' mats having the mold's size were placed in the cavity (30). The mold was tightly closed and a vacuum of 725 mm of mercury was created in the cavity through the vent port (40) connected to an aspirator placed on a tap. At this point the fibres were dried for 2 hours by circulating water at 55 C. The mold was then cooled down with cold water (50). In the meantime the resin was mixed with the initiator and placed in the injection pot (10). From there the resin was injected (20) in the mold with compressed air at a constant gauge pressure of 2.00x105 Pa. This pressure was kept constant in the pressure pot by continuously adjusting manually the compressed air valve. The injection time of course varied with the amount of fibers present in the mold. Once the resin was observed at the outlet, the vent port (40) was closed. A small flask was placed between the vent port and the tap for safety, to prevent any resin from flowing to the tap water. The resin was left flowing at the inlet for 5 more minutes to make sure that the mold was filled completely. Then the inlet ports (20) were closed as well and hot water at constant temperature was circulated in the mold (50). The composite was cured under these conditions for an hour. Meanwhile the injection pot and all the tubes were cleaned with acetone to avoid any clogging due to cured resin.
Example 2:
In this study the polymer used was Derakane[TM] 8084, epoxy vinyl ester resin obtained from the Dow Chemical Co14. and it contained 45 wt% dissolved styrene. This resin is manufactured for use in closed mold processes such as RTM. It is a low viscosity resin, which starts reacting by addition of an initiator. In this case the chosen initiator was MEKP DDM-9 from Ato-FINA. The resin manufacturer recommends using an initiator . .
concentration between 1.5% and 3% by weight. Therefore three different concentrations were investigated during the pilot study: 1%, 1.5% and 2%. Following this study it was decided that a MEKP concentration of 1.5% should give the best results. To allow for curing to occur at 30 C, the resin was mixed with 0.3% by weight of 6% cobalt naphthenate catalyst (Sigma Aldrich Co.). Additionally, 0.025% of 99% N, N
dimethyl aniline (Sigma Aldrich Co.) was used as an accelerator, while 1.5% of methyl ethyl ketone peroxide (9% active oxygen) was the initiator.
The fibres used in this study were manufactured by Flax craft, Inc. The Bastmat 100 was a 4mm mat made of 67% hemp fibres and 33% kenaf fibres. Hybrid fiber mats consisting of hemp fibres sandwiched between slim glass fibre mats (randomly oriented) were prepared. The mats were pre-press at a high temperature (above 80 C) and at a defined thickness to reduce their spring back behaviour and allow more fibres to be placed in the mold.
Example 3:
A 1% by weight aqueous solution of 3-aminopropyltriethoxysiane was prepared using distilled water. The solution was then poured in a bottle and sprayed on the hybrid fibre mats until soaking them. The mats were left on the bench for 30 min to allow the hydrolysis of the silane. Then the fibres were dried first in an oven at 100 C
for one hour followed by 12 hours at 80 C. Fibres were also separately treated with styrene maleic anhydride copolymer with different molecular weight and maleic anhydride content.
Similar treatment was also carried out with rosin ester and polyethylene-imide (PEI).
Finally, treatment of fibres was also carried out with maleated imide cationic polymers such as styrene maleimide and its quaternary salts with different charge density and molecular weights.
Example 4:
The composites with various fibre contents were prepared using the modified RTM process. Once treated the fibre mats were first placed in the mold (30) and dried under vacuum (40). The water evaporating from the fibres could be observed in the liquid trap connected to the vent port (70). Once the fibres were dried for 2 hours, resin mixed . .
with initiator was injected (20) at a constant gauge pressure of 2.00 x105 Pa.
This pressure was chosen by trial and error to provide the shortest injection time possible coupled with a proper wetting of the fibers (knowing that the maximum gauge pressure in the system should not exceed 2.5 x105 Pa for safety). Then the composite was cured at 40 C for an hour. Finally each part obtained was post cured in an oven at 105 C to ensure complete and homogeneous curing of the polyester matrix.
The mold being entirely made of aluminium the flow front could not be observed during the resin's injection. The injection time increased dramatically with increasing fibre content.
The natural fibber composites manufactured by the RTM process in this work were found to be of good quality. An excellent wetting of the fibres was obtained and the drying of the fibres prior to resin's injection permitted to avoid the formation of small gas bubbles in the part due to water evaporation.
Example 5: (Tensile strength) The tensile strength tests were performed using samples made from parts that reached final degree of cure. Figure 2 presents the tensile strength of Silane treated Stypol composite and Silane treated Derakane composite samples. As expected the tensile strength of the samples increased with change in resin from Stypol to Derakane. A
substantial increase was observed between the Stypol resin and the Derakane sample with 20.6% fibers, from 54.86 MPa to 64.52 MPa. After break, very less fiber pull out could be observed on the specimens with Derakane, proving that the fiber-matrix adhesion was substantially improved. For information the tensile strength of a glass fiber/unsaturated polyester composite i.e. Stypol of similar volume fraction and prepared using the same process was added in Figure 2 as well. It can be seen that the natural fibre composites manufactured in this work have tensile strengths approximately 20% lower than their glass fiber/Stypol counterpart.
Example 6 (Tensile modulus) The tensile modulus of the Silane treated Stypol composite and Silane treated Derakane composite samples are shown in Figure 3. The Stypol hemp fibre composites have the same tensile modulus as the Derakane. Once again the glass fiber/Stypol results were added for information showing that the Derakane fibre composites had tensile modulus very close to the synthetic fibre.
Example 7 (Flexural strength) The flexural strengths are reported in Figure 4. The Figure shows the same trend as the results of tensile strengths. The flexural strength of the samples increased with change in resin from Stypol to Derakane. An increase was observed between the Stypol resin and the Derakane sample with 20.6% fibers, from 129.96 MPa to 132.76 MPa. The explanations for these results are that the flexural properties are influenced by the fiber/matrix interface interaction as well. Once again the strength value for the glass fiber sample is greater than that for the natural fiber composites.
Example 8 (Flexural modulus) The modulus results can be seen in Figure 5. The flexural modulus for Derakane composites exhibits similar modulus when compared to Stypol composite.
Example 9 (Impact properties) Table 1 gives the summary of tensile, impact and flexural properties of composites containing 20 to 30% hemp fibre. In this work the manufacturing of natural fiber composites using a modified Resin Transfer Molding was investigated. The drying process before the resin's injection permitted to obtain a good wetting of the fibers as well as to avoid any formation of gas bubbles during curing. In this work data concerning the curing behaviour of Derakane 8084, an epoxy-vinyl ester resin were presented. In order to achieve high fiber contents with hemp fibres in a process such as RTM
the need of pre-pressing stage at 100 C was asserted. This additional step reduced greatly the spring back behaviour of the fibres, making the closure of the mold much easier.
The natural fiber composites obtained by this process were found to be of high quality.
No voids could be observed within the parts. The tensile, flexural properties were found to increase with change in resin from Stypol to Derakane. It was observed that the . .
optimum properties were not reached in this study and that the fiber content higher than 20 vol% should yield better mechanical properties.
The highlights of this invention could be further summarized as:
- The technique has been designed for optimization of RTM
process to achieve the composites with high mechanical performance from hemp/glass fibers and synthetic thermoset resin. The resin injection pressure and temperature have been optimized.
- The curing system has been standardized by selecting appropriate resin, catalyst, accelerators, retarders, coupling agent with their concentrations - It has been observed that the composite with 26% hemp fiber and 7% glass fiber exhibits the optimum tensile strength of 75MPa, Flexural strength of 187 MPa and modulus of 8GPa and impact strength of 200 J/m.

Claims (17)

1. Claim 1: A
   process to prepare structural composites for automotive, aerospace, furniture and sports goods applications whereby composites are manufactured by modified resin transfer molding process, whereby fibres of natural origin are placed in layers inside the cavity of the mold (30), wherein the moisture of fibres first being removed (70) by applying vacuum (40) followed by impregnating resin in liquid form (10) of adequate viscosity, allowing to heat the resin impregnated fibre to solidify the resin inside the mold to obtain product parts of required shape, size, performance and durability requirement.
2. Claim 2. A process in claim 1, wherein the drying of fibre mats are done under controlled vacuum conditions aided by heating of the mold with hot water above 40 C.
3. Claim 3. A process as cited in claim 1, where in the resin system is a thermosetting material may comprise of Silane Modified Polyester resin, Epoxy vinyl ester resin, Silane modified epoxy ester resin, Acrylic resin, silane modified acrylic resin, Soy resin, silane modified soy resin and contains not less than 30 % by weight of the resin composition.
4. Claim 4. A process as cited in claim 1, wherein the said moldable composition comprises cellulosic fibres selected from pulp fibres, bast fibres, leaf fibres, and agricultural waste fibres such as corn, wheat and rice straw and comprises not more than 70 percentage by weight of the said composition.
5. Claim 5. A process as cited in any one of claims 1,3, and 4 where in wood pulp fibres selected from thermomechanical pulp (TMP), kraft pulp and bleached kraft pulp (BKP) from hard wood or soft wood or a combination of the same.
6. Claim 6. A process as cited in any one of claims 1,4 and 5, where in the bast fibres can be selected from hemp, flax, jute and kenaf and / or a combination of the same.
7. Claim 7. A process as cited in claim 1, where in the synthetic fibres comprise of any chemical artificial fibres such as polypropylene, carbon, kevlar, boron, and glass fibres.
8. Claim 8. A composite product made from the process of claim 7, wherein the inorganic fibres can be selected from any chemical artificial fibres such as polypropylene, carbon, kevlar, boron, and glass fibres.
9. Claim 9. A composite product made from the process of any one of claims 4 to 6, wherein the organic fibres are treated with an enzyme or chemicals.
10. Claim 10. A composite product made from the process of any one of claims 4 to 6, wherein the organic fibres are treated or untreated were used in loose form or mat form, more specifically loose form were used in the core and mat form were used on the surface of the layered structure.
11. Claim 11. A composite product made from the process of any one of claims 4 to 7, where in the chemicals used for treatment are from any of the following functional chemicals such as silanes, imides, maleimides, alkanes, alkene dimers, rosin acid and esters, alkyl imines.
12. Claim 12. A composite product made from the process of any one of claims 4 to 8, where in the chemicals used for treatment are Microthene Powder, Polyvinyl acetate, Polyethyleneimine, Styrene Maleic Anhydride, Styrene Maleic Imide, Alkyl ketene Dimer, Rosin Acid, Trimethoxy (3,3,3-trifluoropropyl) silane, (3-Glycidyloxy propyl) trimethoxy silane, Dimethyl (pentafluorophenyl) styryl silane, Chloro-dimethyl (3,3,3-trifluoropropyl) silane
13. Claim 13. A composite product made from the process of any one of claims 4 to 7, where in the enzymes used are from any of the following category: such as endo-glucanase, hemicellulase.
14. Claim 14. A composite product made from the process of any one of claims 7-13, where in the said liquid resin material has been modified with silane chemicals to enhance mechanical properties of the resin in the presence of fibre.
15. Claim 15. A composite product made from the process of any one of claims 7-13, where in the said resin transfer molded composite materials have flexural strength and modulus 140-250% and 300- 700% greater than that of the said resin materials and have flexural modulus exceeding 10GPa.
16. Claim 16. A composite product made from the process of any one of claims 7-14, where in the said composites have impact strength improved by more than 70% by using loose fibre in the layered design.
17. Claim 17. A composite product made from the process of any one of claims 1-16 can be used for structural applications in the automotive, sports goods, furniture and aerospace industry.