EP2160441A2

EP2160441A2 - Mechanically reinforced thermoplastic plastic product for laser-based joining processes

Info

Publication number: EP2160441A2
Application number: EP08784241A
Authority: EP
Inventors: Carlos J. Caro
Original assignee: GRAFE COLOR BATCH GmbH
Current assignee: GRAFE COLOR BATCH GmbH
Priority date: 2007-06-22
Filing date: 2008-06-20
Publication date: 2010-03-10
Also published as: DE102007029239A1; WO2009000252A4; WO2009000252A2; WO2009000252A3

Abstract

The invention relates to the use of plastic products that are characterised by improved mechanical characteristics and are suitable for penetration welding using lasers. The aim of the invention is to provide a novel option for producing a plastic product, which permits a plastic with a higher NIR transparency to be produced using mechanical parameters that are comparable to those of plastics with conventional fillers and using known extrusion or injection moulding methods. This is achieved by the use of plastic products containing at least one nanoscale dispersed filler, said filler consisting of one or more organically modified, natural or synthetic clay mineral (nanoclay, nanolayered silicate).

Description

Mechanically reinforced thermoplastic plastic product for laser-based joining processes

The invention relates to the use of plastic products, which are characterized by improved mechanical characteristics and are suitable for laser transmission welding.

Plastics products are often made in different color variations usually from PP, PA, PC, ABS, POM or associated plastic blends. However, the technically most common primary colors are black and gray in many facets and nuances.

Of all these thermoplastics mentioned, however, polypropylene (PP) and polyamide (PA) are those which most often need to be mechanically reinforced by the addition of inorganic fillers. The need arises from the fact that these carrier polymers are inexpensive on the one hand, and on the other hand can be easily modified by the addition of these fillers to better mechanical properties.

A range of fillers are therefore available to the user for this purpose: aramid and carbon fibers, graphites, carbon nanotubes (CNT), carbon black, talc, kaolins, chalk, barium sulfate, titanium dioxide, glass fiber and glass ball, wollastonite, magnesium carbonate, mica, silica, Wood flour, cross-linked polymers, and many more m.

The addition concentration of the respective fillers depends on economic aspects, i. d. R. according to the physical and chemical properties of the filler itself, the desired mechanical properties of the final product as well as the degree of homogeneous dispersion in the target plastic. From the different types of fillers inevitably results in a different coloration of the final product, depending on whether you z. As black (black), chalk (white) or talc (gray) or glass fiber (colorless) is used.

The most common question in the industry is how to combine two preferably dark colored plastic products with each other, permanently and stably, without renouncing the use of fillers for mechanical reinforcement.

For this purpose, the prior art provides numerous methods, such as bonding, hot riveting and a variety of welding methods (eg high-frequency and microwave welding, ultrasonic welding, plasma (hot gas) welding, induction welding, IR welding, mirror welding, spin welding, Vibration welding and laser transmission welding). For certain applications, the laser transmission welding has proved to be particularly advantageous.

In laser beam welding of two interacting partners, the laser is (or should be) transmitted through the first material and absorbed by the second material. This absorbed energy converts to heat and melts the lower polymer at the interface with the upper laser-transmissive part. The melt as soon as she. cools, then connects both joining partners together. The wavelength of the laser light used may usually be 808, 940 and 1064 nm.

The quality of the weld and the speed of the laser welding process depends u. a. the degree and concentration of the fillers used in the upper laser-permeable as well as in the lower laser-impermeable joining partner.

More filler in the finished plastic part often results in better mechanical properties, but at the expense of laser weldability and process safety. In general, many fillers show a large discrepancy between the required loading concentration for the required mechanical properties (usually 5-30% by weight) and insufficient transmission in the VIS and NIR ranges in order to be used as a transparent joining partner for a laser-based joining process such as laser transmission welding act. Commonly known is the use of glass fibers to improve mechanical properties. Although the laser transmission welding of plastic parts containing glass fibers, as well known in the art, but it can under Circumstances be very difficult to weld two plastic parts, the glass fibers between 15 to 30% by weight and possibly the same colored, to be welded together by a laser beam.

This is due to the fact that each joining partner has to fulfill opposing properties (laser transparency and laser absorption), which can only be achieved with different, coloring, material combinations.

Glass fibers generate high laser reflection. The laser transparency is severely impaired. On the other hand, this reflection reduces the laser power required for the melting process of the absorbing partner. It therefore requires not only a maximum layer thickness of the plastic article, but also the special skills of the user in laser welding to despite high contents of glass fibers (from 15 to 30 wt.%) To offer an economically and technically satisfactory solution.

Furthermore, the particle size and the physico-chemical properties of these fillers make the scattering of the laser light so high that rapid and selective conversion of light into heat along the weld seam can not be achieved. The light energy is scattered and not effectively converted into heat at the interface of the joining partners.

The special properties of the new plastic product are intended to solve the following problems of the prior art:

1. Problem: The relatively high, yet necessary concentration of fillers scatters the laser light very strongly or is impermeable to them. As a result, the laser energy is attenuated and the second laser-absorbing partner does not melt or insufficient. The final product (a dyed plastic product) is thus not permeable enough for a laser beam of a wavelength higher than 800 nm. In contrast, according to this invention reinforced plastic product to scatter little light and selectively transmit the laser light of wavelengths higher than 800 nm, even with dark hiding colors. 2. Problem: The plastic product should have approximately the same mechanical strengths as in the application of conventional fillers, such as. As talc, without loss of NIR permeability or impairment of laser transmission welding.

Initial state in many applications are two plastic parts that are to be welded together. Both parts are usually made from the same starting materials. If two parts made of the same starting materials are laser-welded together, the problem is less with the laser-absorbing joining partner, z. As a dyed with carbon black and talc filled part, but often with the first overlying, possibly dark colored, filled with talc plastic part, which, however, has no laser transparency. This problem does not only occur with dark colored plastics.

While with black plastic parts (dyed with carbon black) at least the laser absorption can be easily achieved, for example, in white parts to be inked neither the laser absorption nor the laser transparency readily accessible.

The color white in the plastics industry is most often and inexpensively produced by the addition of titanium dioxide. It requires relatively low use concentrations in the final product to color either white opaque or to achieve a base color for other light and gray shades. However, titanium dioxide has the disadvantage of greatly scattering the incident laser light during laser beam welding due to its high light refraction. This state allows a general application of titanium dioxide neither as a NIR laser transparent nor as a laser-absorbing joining partner. If, therefore, it is intended to weld two white plastic products to be colored together by means of an NIR laser beam, then titanium dioxide in the usual concentrations must not be used simultaneously in both materials as whitening agent. Other commercially available, known and exotic whiteners, such as zinc sulfide, barium sulfate, barium titanate, zinc oxide and zirconium oxide, are suitable and should therefore be considered in more detail.

Barium sulphate behaves slightly better than titanium dioxide in terms of NIR laser transparency, but due to the high dosages required, it is susceptible to laser beamwelding, as soon as it is desired to use it to mechanically strengthen the plastic part.

At the same time, zinc sulphide exhibits NIR-transparent properties in conjunction with an opaque white coloration as a function of the loading concentration. Here, too, the connection between the loading concentration used, the degree of whiteness achieved and the required NIR laser transparency appears problematic.

Barium titanate is due to its physical and chemical properties as. laser-absorbing partner at relatively high loading concentrations between 0.1 and 5 wt.% Well suited. You can achieve a very good whiteness. However, the raw material costs and the additions make this solution economically uninteresting.

Recently, some of the abovementioned whitening agents in the form of nanoparticles can be used to achieve both opaque white coloration and transparency in the visible (VIS) and NIR ranges. These include in particular zinc oxide and zirconium oxide.

Due to its chemical and physical properties, nano-zinc oxide is already used as a UV absorber with high transparency in the visible (VIS) range alone or in combination with organic light stabilizers in thin food packaging films. It has a high permeability in the visible and NIR range. Nano-zirconia is approximately 10 times more expensive than nano-zinc oxide, but has excellent NIR-transparent properties.

However, our own experiments have shown that, depending on the layer thickness, in some cases a high amount of nanofiller is needed in order to achieve an opaque white coloration. This high loading may result in insufficient NIR laser transparency.

The high loading concentration together with the currently high raw material prices for these nano-minerals cause such an increase in material and manufacturing costs, which ultimately makes the solution of laser welding of white materials technically successful, but can not be exploited commercially.

The addition of carbon black is prior art to achieve a dark coloration, but at the same time achieving the necessary NIR transparency is difficult. Soot-filled plastic parts are usually used exclusively as a laser-absorbing joining partner.

Alternatively, so-called Carbonnanotubes (CNT, Graphitröhrchen) lately. As expected, they come into question as NIR laser-absorbing partners in laser transmission welding. More detailed investigations are described by Larry Dosser, Ken Hix, Kevin Hartke, Rieh Vaia, and Mingwei Li: "Transmission Welding of Carbon Nanocomposites with Direct-Diode and Nd: YAG Solid State Lasers" (2005) black coloring, without regard to laser properties, would be associated with too high a cost.

Even if one assumes that the aforementioned nanofillers would generally lead to the solution of the stated problem, these materials are either currently not available in the form of nanoparticles, their processing and dispersion in the end product too difficult or expensive and ultimately too expensive (factor 10x, 100x or 100Ox) in order to seriously consider an alternative.

Some of these fillers in the form of nanoparticles show a chemical incompatibility with the carrier material, sometimes with the help of Compatibility mediator can be reduced. This often occurring incompatibility between filler and polymer is the cause of the poor dispersion in the carrier material and is later blamed for the missing effects and for the partly high and unnecessary use concentrations.

In summary, many of the standard fillers used for decades (eg, carbon black and titanium dioxide) have not been used to maintain the natural NIR permeability of the plastic for use in laser-based joining processes, such as laser transmission welding, as well as mechanical reinforcement can.

A better solution to the problems of coloring especially dark tones (black, gray) with a good NIR permeability in conjunction with improved mechanical characteristics is therefore urgently needed by the industry.

The object of the invention is to provide a new way of producing a plastic product, which makes it possible to produce a plastic with high NIR transparency with comparable mechanical parameters as in plastics with conventional fillers by known extruder or injection molding. The reinforced plastic product should be NIR-permeable, even if visually dark opaque colors are used according to human standards.

Solution of the task

Although standard fillers such as talc, chalk and barium sulphate would very well mechanically strengthen the final product at usual concentrations, the

Visible (VIS) and near infrared (NIR) transmittance and good laser transparent properties would be lost. Fig. 1 shows clearly what is meant by it.

The maximum achievable NIR transmission of PP injection molded parts, the chalk (E), barium sulfate (D) and talc (F) in the usual addition dosages of 10 and

20% by weight contained not more than 40%. Ultimately, only barium sulfate showed Of all three variants better transmission values than talc and was used only as a reference.

Other fillers are out of the question because of the high laser light scattering, the high laser impermeability, the high application concentrations or simply the high costs.

Surprisingly, organically modified natural or synthetic clay minerals (also known as nanosheet silicates or nanoclays) from the group of the saponites, kenyaites, sepiolites, bentonites, lucenites, montmorillonites, nontronites, beidites, volkonskoites, laponites, hectorites, sauconites, magadites, stevensites and vermiculites in The form of nanoparticles, the so-called organically modified nanoclays (hereinafter also abbreviated to OMN), in nanoscale dispersed form in the final product, the suitable property profile to simultaneously solve the two opposite aspects of the task. On the one hand a good mechanical reinforcing effect and on the other hand a high permeability in the visible but also in the NIR range is achieved.

In the meantime, the use of nanoclays by mass production, worldwide availability, moderate prices, and the concentrations required at the end of the day have considerable advantages over other fillers, even if they were also present in nanoparticle form.

Especially the products of the companies Nanocor (USA), Südchemie (DE), Elementis (USA) and Southern Clay (USA) are generally available and are successfully used to solve many technical problems. These materials are used specifically for the positive - but sometimes also contrary - change of many properties: mechanical properties (strength, impact resistance, stiffness), thermal properties (heat resistance), prevention of dripping behavior during burning (flame retardant) in film and injection molding applications, gas barrier - Property for food film and injection molding applications, abrasion and scratch resistance, UV absorber in films and as a plant-active antibacterial additive. For an optimal processing of nanoclays in the plastic state of the art:

Selection of the Nanoclays, manufacturing and processing parameters for masterbatch or compound and last but not least the right application, for which the best effect can be achieved.

An interesting feature of these materials is that even at high loading concentrations no light refraction phenomena, as with conventional fillers, can be observed. This special feature allows the use of nanoclays in thermoplastics as laser-transparent aggregate using a laser beam especially the wavelengths 808 nm, 940 nm or 1064 nm.

It is important for the characterization of nanoclays in plastics to determine to what extent the individual layers in the final product are separated and dispersed from one another. These processes are known in the literature under the name intercalation and exfoliation. Only good intercalation and exfoliation enable the unfolding of the desired physical and chemical properties of the nanoclays. The layer spacing, which is usually given in Ängström (λ), is a measure of the quality of the dispersion. Typical slice distances may be 10 Å = 1 nm (for unmodified clays) to 40 Å (for exfoliated clays).

To understand the physics of OMN, you have to know their chemistry. The OMN consist of an organic and an inorganic component. The organic content varies in type and concentration among the different OMNs. The ratio between organic and inorganic content may be 20:80, 30:70, 40:60 or 50:50, depending on the manufacturer. These ratios are not to be understood stoichiometrically or quantitatively exactly. It is intended to convey only a picture of how both organic and inorganic parts in the OMN can be related to each other. As an example of the chemical origin of such organic premodifiers, the prior art gives, inter alia: primary, quaternary alkylammonium and Phosphonium ions, aminocarboxylic acids, oxylated and nonoxylated long-chain alkylamines, siloxanes or tallow-based amines.

The inorganic fraction often has as its base either a natural or a synthetic layered mineral from the family of mica, bentonite, saponite, kenyaite, sepiolite, lucenite, montmorillonite, nontronite, beidellite, volkonskoite, laponite, hectorite, sauconite, magadite, stevensite and vermiculite.

The most widely used and commercially available nanodayers belong to the family of smectites, especially bentonites and montmorillonites.

The invention will be explained in more detail with reference to embodiments. The examples describe the overall process and the most advantageous materials in order to be able to demonstrate the solution comprehensibly using the example of polypropylene.

A distinction is made between a) addition of the OMN into the plastic "in situ" in the polymerization reaction or into a finished plastic granulate or b) stepwise provision of the OMN in the form of a masterbatch or compound before the actual production of the final plastic products.

The precursor as a masterbatch or compound results in better dispersing effects of the nanosheets than the direct addition of the nanodayers into the plastic in order to subsequently produce the end product (film, article etc). This necessary procedure is also similar to previous observations in the dispersion and processing of carbon nanotubes (CNTs) in thermoplastics.

For the production of a first masterbatch (MB1), therefore, 40% of a montmorillonite in the form of a powder which has undergone preliminary organic surface modification (for example the product Nanomer I 44 PA from Nanocor USA) is used, 20% of a

Maleic anhydride grafted PP (usually containing 1.5% maleic anhydride). Graft content) as a compatibilizer and 40% of a common PP as the last component.

This mixture is processed by extrusion in a conventional twin-screw extruder at the usual PP processing temperatures, the melt is cooled immediately after emerging as a strand through a water bath and the granules obtained by cutting the cooled melt strand in a granulator. Lenticular or spherical granules, instead of cylindrical granules, can be obtained by the application of underwater granulation. Instead of a twin-screw extruder, a horizontally oscillating co-kneader can also be used.

Subsequently, 20% by weight of this first masterbatch MB1 is added to 80% of a customary PP for injection molding applications, producing normalized tensile specimens which can be used to determine mechanical characteristics. The use concentration of nanoclays in the final plastic product for an NIR laser-based joining process, such as laser transmission welding, was therefore 8% by weight.

On these injection-molded parts, the UV-VIS transmission spectra were measured using a conventional UV spectrophotometer in transmission mode to quantify the NIR laser transmission.

Fig. 2 shows the changes in transmission behavior when 1 (A) and 10 wt% (B) of the montmorillonite nanoclay were incorporated in the PP. It is very easy to observe the high values (> 70%) for the transmission above 800 nm (in the NIR range), even if the concentration of filler has been increased tenfold.

In order to investigate the effects of colors, 2% by weight of a second masterbatch MB2 on the same PP basis were added to this, which, due to the corresponding pigment combination and dosage, had a sufficient amount of both NIR transmission must have over 800 nm and at the same time the end product black color.

This type of special masterbatch is state of the art. The masterbatches can be obtained commercially in any amount from many masterbatch manufacturers.

For the in-house production of this masterbatch MB2, a mixture of blue (PB 29), yellow (PY150) and violet (PV23) can be used in addition to the carrier material PP and the usual dispersing aids, each pigment being mixed with 10% by weight in masterbatch MB2 ,

The Masterbatch MB2 is supposed to bring only the black color and a high laser transparency, but does not contribute to the mechanical reinforcement of the final product. This masterbatch MB2 must therefore contain neither carbon black, nor titanium dioxide, nor barium sulfate, since otherwise the NIR transparency would be lost.

When it comes to laser transmittance above 800 nm, Figure 3 shows the big difference between black colored parts containing 8% nanoclays (G) and 20% barium sulfate (H) in PP. The parts with 8% Nanoclays had better mechanical properties than those with barium sulfate.

4 shows, in addition to the spectrum of the original PP (K), the spectrum of an injection-molded wafer (as a final plastic product) with 8% nanoclays (L) alone and the almost coincident spectra of black-colored nanoclays-reinforced (M) and unreinforced ( N) platelets. This shows once again how, despite having been dyed with a black color masterbatch, the NIR-transparent properties of the plastic product can be obtained by using the nanoclays.

It has been observed that not all nanoclays have the same optical spectral behavior. Illites are non-expandable mica-based multilayer silicates. Injection molded parts of montmorillonite and of as chemical origin were produced at the same content and the UV-VIS-NIR spectrum was measured. The showed Parts of montmorillonite (A) higher in transmission than parts containing illite-based nanoclays (C). This is illustrated in FIG. 5.

When using different OMNs, which differ in type and proportion of organic constituents, it was also observed that such products with a higher proportion of organic premixing agents in Nanoclays tended to show better mechanical properties than those containing lower levels.

This behavior was observed by processing two different OMN (proportion of organic premixer 30% and 45%) at different concentrations in PP (2.5%, 5% and 7.5%) and mechanically loading the test specimens produced from them.

It turned out that the OMN with 45% organic content showed higher strength, stiffness and better impact resistance than the OMN with 30% organic premixer. The increase in some mechanical characteristics in the final product is also associated with the increase in the concentration of OMN, but the higher additions (from 2.5 to 7.5%) did not particularly affect the impact strength and elongation behavior within a series. Compared to the original and unreinforced PP even no significant change in the notched impact strength was found.

The following mechanical property values were measured on standardized tensile test specimens in accordance with customary DIN regulations for the determination of mechanical parameters on thermoplastic plastic parts:

After the new material combination with 8% OMN has withstood the comparison with 20% talc from the viewpoint of the mechanical properties, and the UV-VIS-NIR transmission spectra have a better NIR transmittance of> 40% (see FIGS. 1 to 5), In the laser transmission test, it is important to check whether this combination of materials can ultimately be used as a laser-transparent joining partner in the field trial.

For this purpose, two planar plastic parts 1 and 2 made of PP are used, of which plastic part 1 is the laser-transparent joining partner, and plastic part 2 should be the laser-absorbing joining partner.

Plastic part 1 contains as filler for the mechanical reinforcement 8% of organically modified nanoclays (eg Nanomer I 44 PA Fa. Nanocor) and - if necessary - 2% of a black, NIR-transparent masterbatch MB2 for coloring the plastic part 1. Joining partner 2 contains carbon black for blackening and as a laser-absorbing additive (with a maximum of 1%) and, if necessary, 20% talcum for mechanical reinforcement.

Plastic part 1 is placed on plastic part 2. Through the plastic part 1 through a laser beam of 808 nm or 940 nm wavelength is directed to the interface of both plastic parts. The laser power is limited to a maximum of 100 W, the

Traversing speed up to 10 mm / s, focal length to 100 mm in focus, the

Optics set to point.

The duration of this welding process is no more than five seconds. Subsequently, the result of the two plastic parts welded together is examined. Both the appearance (the seam should have no bubbles) and the quality of the

Weld seam (the seam should withstand high mechanical loads) is an indication of a successful welding process.

As a result of the test, the seam shows no bubbles and all attempts to break apart the connection between the two plastic parts 1 and 2 by hand, fail. Also the continuous mechanical load by a

Tensile testing machine leads only to breakage of high tensile forces (30 N / mm ² ) both plastic parts * 1 or 2, but not to break along the interconnecting weld.

As essential to the invention here is the use of NIR laser transparent nanoclays to name the mechanical reinforcement of thermoplastic products, which can be on the finished plastic product (a mechanically reinforced and NIR-transparent plastic part 1) even a modern laser-based joining method, such as laser transmission welding, apply.

Furthermore, the solution shown here offers the user the opportunity to color the final plastic products according to the respective requirement. A limitation is that such colorants that adversely affect the NIR laser transparency and the incidence of the laser beam must not be used.

The solution of this problem is not possible if one uses natural clay minerals (simple clays) entirely without organic chemical pre-modification.

Claims

claims

1. Use of plastic products containing at least one nanoscale-dispersed filler, which consists of one or more organically modified natural or synthetic clay mineral (Nanoclay,

Nano sheet silicate) from the group of

Saponites, Kenyaites, Sepiolites, Bentonites, Lucenites, Montmorillonites, Nontronites, Beidellites, Volkonskites, Laponites, Hectorites, Sauconites, Magadites, Stevensites and Vermiculites exist as both visible (VIS) and near infrared (NIR) laser transparent material Laser transmission welding as well as

Improvement of mechanical characteristics of the finished end products.

2. Plastics products according to claim 1, characterized in that the nanoscale dispersed filler is contained in the plastic product in a concentration range between 0.5 and 10% by weight.

3. Plastic products according to claim 1, characterized in that the nanoscale-dispersed filler is a montmorillonite nanoday.

4. Plastic products according to claim 1, characterized in that the nanoscale-dispersed filler is a bentonite nanoclay.

5. Plastics products according to claim 1, characterized in that is colored before or after the process of laser transmission welding, while maintaining the NIR permeability and the desired mechanical

Properties.

6. A process for the production of plastic products according to claim 1, characterized in that the preparation of the precursor granular masterbatches or compounds by means of a horizontally oscillating co-kneader.

7. A process for the production of plastic products according to claim 1, characterized in that the preparation of the precursor granular masterbatches or compounds by means of a twin-screw extruder.

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