POLYMER-BASED COMPOSITE MATERIAL WITH IMPROVED MECHANICAL PROPERTIES

Technical Field of the Invention

The present invention relates to polymer-containing composite materials reinforced with a cellulose-based reinforcement material and a filler.

Background of the Invention

Polymer composites are materials made through incorporating one or more different types of components into polymer-based materials and combining the mixture at the macro level. A polymer composite mainly comprises a polymer matrix and reinforcement components disposed in the matrix. The reinforcement material is usually responsible for increasing the mechanical strength of the polymer and/or imparting the polymer a new functional property. Currently, polymer composites are still commonly achieved using petroleum-based polymer materials, but the use of such conventional polymers results in the emission of excessive amounts of CO ₂ into the environment. More importantly, these polymers that are known to be resistant to degradation in nature, if mixed into water resources, air and soil, expose the entire ecosystem to toxic effects, with a knock-on effect for many years.

On the other hand, biodegradable polymers are polymers that can degrade in nature due to the fact they are produced from biologically-based and therefore renewable resources. With these features, they make contribution to recycling without having toxic effects on nature and living beings in the short or long term. This allows the provision of environmentally-friendly alternative materials to petroleum-based plastics, which can be produced from a limited number of resources and do not degrade in nature.

As in all polymer materials, it is possible to improve the mechanical properties of biodegradable polymers with various reinforcement materials and fillers and to thus vary and enhance their area of use, and there is continuing research worldwide on the development thereof. Reinforcement materials and fillers can have granular or fibrous form. Organic and/or inorganic components of different geometries such as flakes, pellets, granules, chips are used as granular reinforcement materials. Clays, metal oxides, synthetic fibers, biofibers, glass fibers and cellulosic fillers are examples of reinforcement materials used in the art.

One of the criteria that determine the mechanical properties of the final composite material is to define which component will be used as the reinforcement material and in which structure and geometry. In this regard, reinforcement materials are selected taking into consideration the desired mechanical and functional properties of the composite material. Another criterion to be considered when selecting reinforcement materials and fillers should be the impact on human health and the environment. For example, the flammability of a polymer material is restrictive for the polymer industry, limiting the area of use of the material. On the other hand, the additives that are used to provide flame retardant effect are harmful to the environment due to their toxic content. Such paradoxes are often experienced in the plastics industry. This results in the production of composite materials with reduced, but not completely eliminated environmental damage, also not offering the desired quality.

For instance, a biopolymer-based composite material and production method are mentioned in the patent document WO2004113435A1 in the state of the art. It proposes a composition in which said composite material is reinforced with a plantbased reinforcement component and a thermoactive material. The plant-based reinforcement component is obtained by fermentation of such components as crop straws, wood and grains. It is preferred that the thermoactive material is in the form of raw biopolymer pellets. The aim therein is to produce composite materials that are completely biologically based and therefore biodegradable in nature.

However, another problem in the art is the problem of compatibility between the polymer material and the fillers. This problem results in additional processing steps and therefore additional costs. As in the example above, said difficulty is also experienced in the clay-containing polymer composites in the current art. Since the clay-containing fillers are inorganic, they need to be subjected to certain modification processes before being mixed into the organic polymer matrix. Given that such modification processes involve chemical treatment and similar processes, they cause negative impacts on the human health and the environment, also increasing the cost and duration of the process. Due to said structural incompatibility, unmodified fillers can be loaded into the polymer matrix in too low amounts to ensure the required functionality in the resulting composite material.

In summary, considering the improvements in the state of the art in this field, there is still a need for fillers and reinforcement materials that are perfectly compatible with the matrix, as well as polymer composite materials having thus increased mechanical strength and being environmentally friendly.

Objects of the Invention

The main object of the present invention is to eliminate the above-mentioned deficiencies and disadvantages of the prior art.

Another object of the present invention is to obtain polymer-based composite materials with increased mechanical and thermal strength and enhanced area of use.

Another object of the present invention is to obtain polymer-based composite materials exhibiting flame retardant properties without using additives that are harmful to nature and human health.

Another object of the present invention is to obtain polymer-based composite materials with increased content of natural and/or biodegradable components, preferably having a fully biodegradable content.

Still another object of the present invention is to provide a method for obtaining polymer-based composite materials as described above, which allows a high load of fillers and/or reinforcement materials in the polymer matrix, and which does not require pretreatment and thus reduces the cost of production.

Summary of the Invention

The present invention provides polymer-based composite materials reinforced with a cellulose-based reinforcement material and vermiculite content. The synergistic effect of the said hybrid structure provides surprisingly improved mechanical and thermal properties in the final composite material. According to a preferred embodiment, said vermiculite material preferably has a flaky form. According to this embodiment of the invention, the average particle size of the flaky vermiculite material is in the range of 0.5 pm to 8mm, preferably in the range of 0.1 mm to 3 mm, more preferably in the range of 0.5 mm to 1.4 mm. It is observed that such ranges of ratio increase the synergistic effect.

According to an embodiment, said cellulose-based reinforcement preferably has a fibrous form. In a preferred embodiment, the weight ratio of the cellulose-based reinforcement material to vermiculite is in the range of 1:30 to 30:1, preferably 1:5 to 5: 1, more preferably 1: 1 to 3: 1.

According to an embodiment, said cellulose-based reinforcement material is obtained by recycling waste cellulose. In a more preferred embodiment, the waste cellulose includes cellulose-based textile waste. In this way, the composite material of the invention gains a cost-effective and environmentally friendly quality, also making contribution to recycling.

In the preferred embodiment of the invention, the polymer matrix used in the polymer- based composite material of the invention is selected from natural or synthetic biodegradable polymers. It is observed that the synergy arising from the combination of a cellulose-based reinforcement material and vermiculite works effectively in the biodegradable polymer matrices. This enhances the environmentally-friendly characteristic of the inventive polymer-based composite material. According to the most preferred embodiment of the invention, the polymer matrix comprises PLA.

The invention also provides a method for obtaining the polymer-based composite material according to the invention, comprising a thermokinetic mixing step.

Brief Description of the Drawings

Figure 1 - A SEM image of a flaky (laminar) vermiculite particle in the composition of a polymer-based composite material according to an embodiment of the invention.

Figure 2 - A SEM image of a mixture of a PLA-based composite material comprising 22.5% by weight of cellulose-based reinforcement material and 7.5% by weight of vermiculite, according to an embodiment of the invention. Detailed Description of the Invention

The invention mainly describes a polymer-based composite material which is reinforced with a cellulose-based reinforcement material, also comprising vermiculite as a filler.

Vermiculite is a phyllosilicate mineral in the form of clay that mainly contains Al, Si, Mg elements, which undergoes significant expansion when heated. In a preferred embodiment of the invention, said vermiculite comprises combinations of multiple components selected from SiO ₂ , AL ₂ O ₃ , MgO, Fe ₂ O ₃ , K ₂ O, and CaO. According to an embodiment, vermiculite contains 10-60% SiO ₂ , 5-30% AL ₂ O ₃ , and 5-30% MgO by weight. In another embodiment, in addition to these components, it may also comprise 5-30% Fe ₂ O ₃ , 1-10% K ₂ O, 1-10% CaO by weight.

Vermiculite, with its expandable and therefore water-retaining structure, provides insulation, lightness and non-flammability in the material it is used in. Due to its structure, it can be used in many fields from agriculture to construction sector. One of the advantages to use vermiculite in the present invention is that it can be used in the polymer-based composite material of the invention without being subjected to any chemical treatment, though it is inorganic in nature.

The flame-retardant effect of vermiculite and accordingly, its use as a fire-retardant in structural mortars, notably in the construction industry, are already known. In the studies carried out in relation with the present invention, vermiculite content was included in order to impart flame-retardant properties to the polymeric composites and accordingly, to expand the usage areas of these composites. In the studies carried out within the framework of the present invention, it has been observed that vermiculite increases mechanical and thermal strength and also exhibits flame retardant effect in polymer-based composites. Moreover, it has been found that vermiculite, especially when used as a filler in polymer-based composite materials containing cellulose-based reinforcement materials, shows a synergistic effect with the cellulose-based reinforcement material, thereby surprisingly improving the thermal and mechanical strength.

In an embodiment of the invention, vermiculite is thermally treated and disintegrated into the desired geometry and size according to the structure to be used in the matrix, or it is provided in the desired geometry and particle size in the thermally treated state. In the preferred embodiment of the invention, the polymer-based composite material of the invention comprises vermiculite particles having a flaky (laminar) structure. The flaky structure of the vermiculite particles is considered to yield more effective results in increasing the mechanical and thermal strength of the final composite material as it increases the contact surface and interfacial interaction in the polymer matrix. Said flaky (laminar) structure can be seen in detail in the SEM image illustrated in Figure 1.

According to this embodiment of the invention, the average particle size of the flaky vermiculite material is in the range of 0.5 pm to 8mm, preferably in the range of 0.1 mm to 3 mm, more preferably in the range of 0.5 mm to 1.4 mm. Here, "particle size" refers to the length, width or thickness of the vermiculite particle. The "average particle size" is the average value of the particle size of the vermiculite particles in a cumulative distribution. A differential sieve analysis is used to calculate the average particle size.

According to a preferred embodiment of the invention, said cellulose-based reinforcement material has a fibrous form. The fibrous reinforcement material establishes an interfacial connection with the composite polymer matrix and assists the matrix to maintain its structural integrity. Therefore, it improves the mechanical properties of the final composite material, such as tensile and flexural strength, while resulting in lightness in the material. Within the scope of the present invention, the fibrous cellulose-based reinforcement material, which is included in the polymer matrix along with vermiculite, shows a synergistic effect and allows to obtain much more durable final products in mechanical and thermal terms.

In an embodiment of the invention, the weight ratio of the cellulose-based reinforcement material in the polymer-based composite material is selected from the range of 1-90%, preferably 5-50%, more preferably 5-30%.

In a corresponding embodiment or another embodiment of the invention, the weight ratio of vermiculite in the polymer-based composite material is selected from the range of 1-90%, preferably 5-50%, more preferably 5-30%.

According to an embodiment of the invention, the ratio of the cellulose-based reinforcement material to vermiculite in the composite material according to the invention is in the range of 1:30 to 30: 1 by weight. This ratio is preferably selected from 1:5 to 5:1, more preferably from 1:1 to 3:1. It is found that all the ratio ranges given herein serve the purpose of the invention. However, it is observed that the synergistic effect between the cellulose-based reinforcement material and vermiculite increases especially at the ratio between 1: 1 and 3: 1, thus resulting in superior final products in terms of mechanical and thermal properties.

One of the objects of the invention is to increase the mechanical and thermal strength of the polymer composites while providing environmentally-friendly alternative solutions with the components used. Due to their structure, cellulose-based reinforcement materials can disappear in nature, i.e., they are biodegradable.

In an embodiment of the invention, said cellulose-based reinforcement material is supplied from cellulose-containing materials obtained by recycling of the waste cellulose. More preferably, said waste cellulose comprises cellulose-based textile waste. Here, the term "cellulose-based" is used for materials containing at least 50% cellulose by weight. For example, said cellulose-based textile waste may be made of 100% cotton.

Normally, textile waste is transported to areas far from the production area for incineration purposes, which results in separate costs for both transportation and incineration. In this regard, the use of cellulose-based textile waste in the invention contributes to the reduction of costs and environmentally friendly recycling.

In an embodiment of the invention, the polymer-based composite material comprises at least one polymer matrix selected from polylactic acid, polypropylene, polyethylene, polyethylene terephthalate, acrylonitrile butadiene styrene, polybutylene succinate, polymethyl methacrylate, poly styrene, polyvinyl chloride, polyacrylonitrile, polybutylene terephthalate, styrene acrylonitrile, polyglycolic acid, polyorthoester, polyphosphoester, polyanhydride, polyester amide, polyamide, polycaprolactone, polyClactic-co-glycolic acid), poly(33-hydroxybutyric acid), polybasic acid, polyadipic acid, polyphosphazene, polydioxanone polymers.

In a preferred embodiment of the invention, the polymer-based composite material comprises at least one polymer matrix selected from natural or synthetic biodegradable polymers. In a more preferred embodiment, said biodegradable polymers are selected from the group comprising polylactic acid, polyglycolic acid, polyorthoester, polyphosphoester, polyanhydride, polyester amide, polyamide, polycaprolactone, polyClactic-co-glycolic acid), poly(33-hydroxybutyric acid), polybasic acid, polyadipic acid, polyphosphazene, polydioxanone.

According to the most preferred embodiment of the invention, the polymer-based composite material of the invention comprises polylactic acid (PLA) as the polymer matrix. The studies carried out within the scope of the invention have shown that biodegradable polymers, preferably PLA, as a polymer matrix, further enhance the synergistic effect between the cellulose-based reinforcement material and the vermiculite, when compared to polypropylene (PP). The SEM image illustrated in Figure 2 demonstrates that the PLA matrix, the cellulose-based reinforcement material and vermiculite exhibit perfect compatibility and form a homogeneous composite.

In order to examine the synergistic effect between the cellulose-based reinforcement material and vermiculite in the polymer-based composites, which is put forward within the scope of the invention, samples containing only vermiculite in the polymer matrix and those containing only cellulose-based reinforcement material in the polymer matrix were compared with the exemplary compositions containing both vermiculite and cellulose-based reinforcement material in the polymer matrix. For this purpose, composition samples were prepared at different ratios and these samples were subjected to the tests for tensile strength, modulus of elasticity (Young's modulus), flexural strength and flexural modulus. Said tests were carried out individually for both PLA-containing polymer matrix and PP-containing polymer matrix, in order to observe the effect of using biodegradable polymers. A pure polymer material free of vermiculite and cellulose-based reinforcement material was used as the control group.

The results of the mechanical strength tests performed with a polymer matrix containing polylactic acid (PLA) are presented in Table 1 below.

Table 1. Mechanical strength of the PLA-based composites

*CBRM: cellulose-based reinforcement material

The results of the mechanical strength tests performed with a polymer matrix containing polypropylene (PP) are presented in Table 2 below.

Table 2. Mechanical strength of the PP-based composites

*CBRM: cellulose-based reinforcement material

An examination of the test results reveals that the use of vermiculite alone or the cellulose-based reinforcement material alone in the polymer-based composite materials enhances the mechanical properties to some extent. On the other hand, it is demonstrated that the hybrid use of vermiculite and cellulose-based reinforcement material exhibits a greater effect than either component exhibits alone. Moreover, it is noted that the mechanical properties are remarkably improved by means of a triple synergistic effect resulting from the use of the two components together in a biodegradable polymer matrix, especially in a PLA-based matrix. In fact, it is noticeable that this effect is much higher compared to the improvement in PP-based polymer composites.

In addition to these tests, the effects of vermiculite and cellulose-based reinforcement material on thermal strength were also examined. Accordingly, heat deflection temperature (HDT) and thermogravi metric analysis (TGA) tests were carried out. The results of the HDT test performed with a polymer matrix containing polylactic acid (PLA) are presented in Table 3 below.

Table 3. Heat deflection temperature of the PLA-based composites

*CBRM: cellulose-based reinforcement material The results of the TGA tests performed with polymer matrices containing polylactic acid (PLA) and polypropylene (PP) are presented below in Table 4 and Table 5, respectively. TGA is an important parameter especially in measuring the flame- retardant effect. If the degradation temperature decreases, the flame-retardant effect takes place before the ignition (flaming).

Table 4. Thermogravimetric analysis results of the PLA-based composites

*CBRM: cellulose-based reinforcement material

Table 5. Thermogravimetric analysis results of the PP-based composites *CBRM: cellulose-based reinforcement material

Finally, differential scanning calorimetry (DSC) tests were performed in order to measure the degree of crystallization of the composite materials containing vermiculite and cellulose-based reinforcement materials, and the results are presented in Table 6 and Table 7 for the PLA- and PP-based compositions respectively.

Table 6. Results of the differential scanning calorimetry analysis of the PLA-based composites

*CBRM: cellulose-based reinforcement material

Tm: Melting point

Tg: Glass transition point

Specific heat capacity value of 100% crystalline PLA in the literature: 93 J/g

Table 7. Results of the differential scanning calorimetry analysis of the PP-based composites

*CBRM: cellulose-based reinforcement material

Tm: Melting point

Specific heat capacity value of 100% crystalline PP in the literature: 209 J/g

It is found that the combined use of vermiculite and cellulose-based reinforcement materials in both PLA- and PP-based composites significantly increases crystallinity. This demonstrates that there is an improvement at a higher rate in the mechanical properties, especially with the synergistic effect resulting from such hybrid use.

The present invention also encompasses a method for the production of the polymer- based composite material according to the invention. Said method mainly comprises the state-of-art techniques for forming polymer composites, but also comprises a thermokinetic mixing step in order to achieve and/or increase the above-mentioned synergistic effect. This is where the originality of the invention lies. In the preferred embodiment of the invention, said thermokinetic mixing is carried out at a mixing speed in the range of 4000-5000 rpm. According to this embodiment, the preferred mixing temperature is selected from the range of 170-220°C and the mixing time is selected from the range of 10 to 150 seconds, preferably 30 to 90 seconds.

Thanks to thermokinetic mixing, very high mixing speeds can be achieved in a short period of time, and even two incompatible materials can be mixed homogeneously due to the high shear force. Within the scope of the invention, it is possible to mix both the filler vermiculite and the fiber cellulose-based material without leaving any cavity in the polymer matrix. Thus, incompatibility of the materials can be completely eliminated while increasing the synergistic effect. Moreover, the thermokinetic mixing step offers advantages in terms of cost and lightness in that it allows the use of high ratios of fillers and reinforcement materials in the polymer matrix, and increases the rate of improvement in mechanical properties. So, an improvement rate of more than 200% is observed in the mechanical properties.

In the method of the invention, cellulose-based reinforcement material and vermiculite are first added at the determined ratios and mixed in the polymer matrix. Then a thermokinetic mixer is used in order to obtain a molten mixture. The thermokinetic mixer is preferable to conventional melt blending methods such as in-line extrusion, especially for obtaining polymer composites containing natural fibers such as cellulose. Thermokinetic mixing, when compared to the extrusion technique, has a high shear stress and very low processing/residence time. The main advantage of using a thermokinetic mixer in the production of the polymer-based composites according to the invention is that it can produce products in a very short period of time (10-150 s) and at a low temperature (170-220°C). The most important parameters to be taken into consideration for the production of high-quality products are the decreased temperature and reduced processing time. High temperatures and long processing times lead to the degradation of both the polymer matrix and the reinforcement materials used, thereby losing the mechanical and functional properties thereof. In addition, the processing time reduced by the thermokinetic mixing step and the ability to process at low temperatures are also among the factors that reduce the cost of production.