Process for twin-sheet forming high heat distortion temperature thermoplastic material and articles therefrom

A method for twin sheet forming layers high heat distortion temperature thermoplastic materials. The process employs a low heat distortion temperature thermoplastic material layer disposed between the layers of high heat distortion temperature material layer to permit adhesion therebetween at moderate temperatures to reduce thermal degradation. The structures made therefrom are useful as aircraft ducting.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process for twin sheet forming high heat 
distortion temperature thermoplastics and structures made therefrom. 
2. Description of Related Art 
Twin sheet forming processes are known in the art. (See, for example, 
Dresen, et. al., U.S. Pat. No. 4,428,306) Twin sheet forming processes are 
useful in producing rigid structures having hollow portions for reduced 
weight and increased strength. Prior twin sheet forming processes have 
employed low heat distortion temperature thermoplastics which, while being 
suitable for the manufacture of many items, are unsuitable for 
applications such as aircraft ducting which require the use of 
thermoplastics having good flame resistance, many of which have high heat 
distortion temperatures. While these known processes for twin sheet 
forming are suitable for producing articles from low melt temperature 
thermoplastics, these prior processes are typically not suitable for 
single layer high heat distortion temperature thermoplastics because the 
molds used therein if heated to a temperature high enough to achieve 
bonding between the sheets are too hot to permit the sheets to solidify 
into the desired shapes. While specialized molds employing high 
temperature adhesion zones and low temperature forming zones can be 
employed, such specialized molds add unnecessarily to the equipment costs, 
and the adhesion zones, usually the clamped mold peripheries, have had to 
be maintained at temperatures well above the heat distortion temperature 
of the sheets. 
Accordingly, one object of the present invention is to provide a process 
for producing twin sheet formed structures from high heat distortion 
thermoplastic materials wherein the mold temperatures are below the heat 
distortion temperature of the high heat distortion temperature materials. 
Another object is to provide structures from twin sheet formed 
thermoplastic materials having high heat distortion temperatures. 
SUMMARY OF THE INVENTION 
The present invention involves a process for twin sheet forming 
thermoplastic materials having high heat distortion temperatures. The 
process employs a laminate having a layer of high heat distortion 
temperature thermoplastic material and a bonding layer of a low heat 
distortion temperature thermoplastic material. The mold temperatures 
required to promote adhesion between the twin sheets can be well below the 
heat distortion temperature of the high heat distortion temperature 
thermoplastic material. The molds employed in the present invention do not 
require specially heated adhesion zones but can have substantially uniform 
temperatures. 
Twin sheet forming processes are set forth in Dresen, et. al., U.S. Pat. 
No. 4,428,306, which is incorporated herein by reference and which 
involves a twin sheet thermoforming process wherein each sheet is vacuum 
formed and the sheets are fused together as described therein by 
application of pressure between the sheets. 
The twin sheet forming process of the present invention involves placing a 
pair of high heat distortion temperature (HHDT) thermoplastic sheets 
preheated to a temperature above their heat distortion temperature between 
two female mold halves having temperatures below the heat distortion 
temperature of the HHDT material, placing a metal air tube between the 
sheets, clamping the mold halves together about the periphery of the 
thermoplastic sheets, injecting high pressure air, preferably at least 50 
psi, through the metal air tube and between the sheets, and optionally 
pulling vacuum on the exteriors of the sheets to force the sheets to 
spread apart at their centers and conform to the interior configuration of 
the respective mold halves, and cooling the sheets to a temperature below 
the heat distortion temperature of the HHDT material. The articles made 
therefrom are hollow and may be used for any number of purposes including 
ducting after further manufacturing steps. 
More preferably, the twin sheet forming process involves (i) preheating a 
pair of high heat distortion temperature thermoplastic layers to a 
temperature above their heat distortion temperature, preferably preheating 
to a temperature above 450.degree. F., (ii) placing the pair of high heat 
distortion temperature (HHDT) thermoplastic layers between a pair of 
female mold halves having temperatures below that of the heat distortion 
temperature of the HHDT material, the high heat distortion temperature 
thermoplastic layers having a layer of low heat distortion temperature 
(LHDT) thermoplastic material located therebetween, (iii) forcing the mold 
halves together to pinch a substantial portion of the periphery of the 
layers together, (iv) injecting high pressure gas between the HHDT 
material layers to force the HHDT layers apart at their centers into 
conformance with the mold halves, and (v) cooling the layers to a 
temperature below the heat distortion temperature of the HHDT material. 
Suitable high heat distortion temperature (HHDT) materials have a heat 
distortion temperature at 264 pounds per square inch of at least 
300.degree. F. and include thermoplastics such as polyetherimides, 
polyamide-imides, polyimides, polysulfones, polyethersulfone, 
polyphenylsulfone, polyetheretherketone, polyetherketoneketone, polyaryl 
sulfone, aromatic polyamides, polyarylsulfones, 
polyphenyleneether/polystyrene blends and blends of these thermoplastics 
with other thermoplastics. 
The preferred high heat distortion temperature material is polyetherimide. 
Also preferred as the high heat distortion temperature material is a blend 
having 45% by weight polyetherimide, 25% by weight of a 
copolyestercarbonate, 10% by weight Bisphenol-A polycarbonate, and 20% by 
weight of a silicone-polyetherimide. 
Polyetherimides of the families disclosed in Wirth, et. al., U.S. Pat. No. 
3,787,364 and Takekoshi, U.S. Pat. No. 4,024,101, both patents 
incorporated herein by reference, are useful in this invention. Polymers 
of the formula below are particularly useful: 
##STR1## 
wherein R is a divalent aromatic radical containing from 6-20 carbon 
atoms, R' is a divalent radical which is the organic residue of a diamine 
reacted with a nitro-substituted aromatic anhydride and n is an integer 
having a value from about 10, for instance, from 2 to at least 5,000. More 
specifically, R can be a member selected from the group consisting of 
phenylene, lower alkylphenylene, 
##STR2## 
wherein X is a member selected from the group consisting of bivalent 
aliphatic of one to eight carbon atoms inclusive; cycloaliphatic of four 
to eight carbon atoms, inclusive; or araliphatic of seven to ten carbon 
atoms, inclusive; 
##STR3## 
and R' is a member selected from the group consisting of R, xylylene, 
alkylene containing 2-20 carbon atoms, and cycloalkylene of four to eight 
carbon atoms, inclusive. 
Of these compounds, the preferred ones are those wherein R' is a phenylene 
or alkyl substituted phenylene each alkyl of one to three carbon atoms, 
the number of alkyl substituents being one to three and R is 
##STR4## 
wherein X is a single bond, alkylene of two to eight carbon atoms, 
inclusive; alkylidene of one to eight carbon atoms, inclusive; cycloalkyl 
of four to eight carbon atoms, inclusive; cycloalkylidenyl of four to 
eight carbon atoms, inclusive; 
##STR5## 
The preferred high polymer comprises repeating units of the structure 
##STR6## 
The organic portion of the specific diamino compound, R', which can be 
employed in the preparation of the aromatic polyetherimides are 
illustratively exemplified as the diamines at column 3, lines 10-47 of 
U.S. Pat. No. 3,787,364, which is incorporated herein by reference. 
The number of carbon atoms maximum to be used in various alkylene, 
alkylidene, cycloalkyl, cycloalkylidene, etc. groups are eight carbon 
atoms, inclusive unless otherwise stated. 
The preferred polyetherimide has a heat distortion temperature of 
360.degree. F. at 264 pounds per square inch. 
Suitable polyamideimides are set forth in U.S. Pat. Nos. 4,331,799 and 
4,728,697, both of which are incorporated herein by reference. 
Polyamideimides can be prepared by reacting diamines with a mixture of a 
dianhydride and an acyl chloride of a carboxy anhydride or may be prepared 
by reacting a polyamine with a carboxyanhydride and a dianhydride. 
Polyimides have repeating units of the formula: 
##STR7## 
wherein R" is an aliphatic or aromatic divalent hydrocarbon radical. 
Preferably, R" includes aromatic groups. Polyimides are well known in the 
polymer industry. Suitable polyimides are disclosed in Berdahl, U.S. Pat. 
4,746,720 and Heath, et. al., U.S. Pat. No. 3,847,867, both patents being 
incorporated herein by reference. 
Polysulfones are obtained from the condensation reaction of a dihydric 
phenol and a dihalo sulfone, preferably the dihydric phenol is bisphenol A 
and the dihalosulfone is dichlorophenyl sulfone. Polyphenyl sulfones are 
derived from the reaction of dihydric phenol and a dihaloaromatic sulfone. 
Polyether sulfones are also well known in the polymer industry. Suitable 
polysulfones include polyphenylene sulfones. 
Polyetheretherketones and polyetherketoneketones are known thermoplastic 
materials and can exhibit heat distortion temperatures of about 
300.degree. F. under loads of 264 pounds per square inch (PSI). 
Polyetheretherketones are set forth in U.S. Pat. No. 4,673,450 and 
4,629,200, both of which are incorporated herein by reference. 
Polyetherketoneketones are disclosed in Japanese Patent No. 62/044425. 
A particularly useful high heat distortion temperature material is a blend 
having about 50% by weight of a polyetherimide, about 40% by weight of a 
polyestercarbonate and about 10% by weight of an organopolysiloxane 
carbonate as disclosed in Patterson, et. al., U.S. Pat. No. 4,735,999 and 
Vaughn, U.S. Pat. No. 3,189,662, both patents being incorporated herein by 
reference. 
Aromatic polyamides are derived from an aromatic diamine and an aromatic 
dicarboxylic acid. A specific aromatic polyamide may be derived from the 
reaction products of toluenediamine and terephthalic acid and/or 
isophthalic acid. 
The polyphenylene ethers comprise a plurality of structural units having 
the formula 
##STR8## 
In each of said units independently, each Q.sup.1 is independently 
halogen, primary or secondary lower alkyl (i.e., alkyl containing up to 7 
carbon atoms), phenyl, haloalkyl, amino-alkyl, hydrocarbonoxy, or 
halohydrocarbonoxy wherein at least two carbon atoms separate the halogen 
and oxygen atoms; and each Q.sup.2 is independently hydrogen, halogen, 
primary or secondary lower alkyl, phenyl, haloalkyl, hydrocarbonoxy or 
halohydrocarbonoxy as defined for Q.sup.1. Examples of suitable primary 
lower alkyl groups are methyl, ethyl, n-propyl, n-butyl, isobutyl, n-amyl, 
isoamyl, 2-methylbutyl, n-hexyl, 2,3-dimethylbutyl, 2-, 3- or 
4-methylpentyl and the corresponding heptyl groups. Examples of secondary 
lower alkyl groups are isopropyl, see-butyl and 3-pentyl. Preferably, any 
alkyl radicals are straight chain rather than branched. Most often, each 
Q.sup.1 is alkyl or phenyl, especially C.sub.1-4 alkyl, and each Q.sup.2 
is hydrogen. Suitable polyphenylene ethers are disclosed in a large number 
of patents. 
Both homopolymer and copolymer polyphenylene ethers are included. Suitable 
homopolymers are those containing, for example, 2,6-dimethyl-1,4-phenylene 
ether units. Suitable copolymers include random copolymers containing such 
units in combination with (for example) 2,3,6-trimethyl-1,4-phenylene 
ether units. Many suitable random copolymers, as well as homopolymers, are 
disclosed in the patent literature. The polyphenylene ether employed is 
preferably in a form of a blend with polystyrene. 
The LHDT material to be suitable as the bonding layer in the process of the 
present invention preferably has a heat distortion temperature at 264 
pounds per square inch of at most about 280.degree. F. and must adhere to 
the particular HHDT material. While the LHDT materials listed below may 
not be suitable melt adhesive layers for particular HHDT materials, they 
may find utility as melt adhesive layers for other HHDT material layers. 
Suitable low melt temperature materials include polycarbonates, 
polyestercarbonates, aliphatic polyamides, polyesters including 
polyethylene terephthalate and polybutylene terephthalate, and polyolefins 
such as polyethylene, silicon-polyimides and blends of these with other 
thermoplastic resins, including, for example, aromatic 
polycarbonate/polybutyleneterephthalate blends. 
Polyolefins or olefin-based copolymers employable in the invention include 
low density polyethylene, high density polyethylene, linear low density 
polyethylene, isotactic polypropylene, poly(1-butene), 
poly(4-methyl-1-pentene), propylene-ethylene copolymers and the like. 
Additional olefin copolymers include copolymers of one or more a-olefins, 
particularly ethylene, with copolymerizable monomers including, for 
example, vinyl acetate, acrylic acids and alkylacrylic acids as well as 
the ester derivatives thereof including, for example, ethylene-acrylic 
acid, ethyl acrylate, methacrylic acid, methyl methacrylate and the like. 
Also suitable are the ionomer resins, which may be wholly or partially 
neutralized with metal ions. 
The polyesters in the resinous compositions of this invention usually 
comprise structural units of the formula: 
##STR9## 
wherein each of R.sup.1 and R.sup.2 is a divalent aliphatic, alicyclic or 
aromatic radical containing about 2-10 carbon atoms. At least about 30 of 
said units are usually present, with at least about 50 being preferred. 
Such linear polyesters are typically prepared by the known reaction of 
dihydroxy compounds with dicarboxylic acids or functional derivatives 
thereof such as anhydrides, acid chlorides or lower alkyl (especially 
methyl) esters, preferably the esters. 
The R.sup.1 radicals may be one or more aliphatic, alicyclic or aromatic 
radicals, alicyclic radicals being known to those skilled in the art to be 
equivalent to aliphatic radicals for the purposes of the invention. They 
may be derived from such dihydroxy compounds as ethylene glycol, 
1,4-butanediol (both of which are preferred), propylene glycol, 
1,3-propanediol, 1,6-hexanediol, 1,1-decanediol, 
1,4-cyclohexanedimethanol, 2-butene-1,4-diol, resorcinol, hydroquinone and 
bisphenol A. They may also be radicals containing substituents which do 
not substantially alter the reactivity of the dihydroxy compound (e.g., 
alkoxy, halo, nitrile) or hereto atoms (e.g., oxygen or sulfur). 
The R.sup.2 radicals may be derived from such acids as succinic, adipic, 
maleic, isophthalic and terephthalic acids or similar substituted and 
hereto atom-containing acids. It usually contains about 6-10 carbon atoms. 
Most often, R.sup.1 and R.sup.2 are hydrocarbon radicals. Preferably, 
R.sup.1 is aliphatic and especially saturated aliphatic and R.sup.2 is 
aromatic. The polyester is most desirably a poly(alkylene terephthalate), 
particularly poly(ethylene terephthalate) or poly(1, 4-butylene 
terephthalate) (hereinafter sometimes simply "polyethylene terephthalate" 
and "polybutylene terephthalate", respectively) and especially the latter. 
Such polyesters are known in the art as illustrated by the following 
patents: 
______________________________________ 
2,465,319 
3,047,539 
2,720,502 
3,671,487 
2,727,881 
3,953,394 
2,822,348 
4,128,526. 
______________________________________ 
The polyesters preferably have number average molecular weights in the 
range of about 10,000-70,000, as determined by gel permeation 
chromatography or by intrinsic viscosity (IV) at 30.degree. C. in a 
mixture of 60% (by weight) phenol and 40% 1,1,2,2-tetrachloroethane. 
The aliphatic polyamide may be obtained from the reaction of an aliphatic 
diamine and an aliphatic dicarboxylic acid and/or a 
monoaminomonocarboxylic acid. Suitable polyamides have repeating units of 
the formula selected from the group consisting of: 
##STR10## 
wherein R is an aliphatic or cycloaliphatic divalent hydrocarbon radical. 
Suitable aliphatic polyamides are set forth in Gallucci, et. al., U.S. 
Pat. No. 4,749,754 and Sybert, U.S. Pat. No. 4,732,937, both patents are 
incorporated herein by reference. 
Silicone-polyimides are set forth in Cella, et. al., U.S. Pat. No. 
4,690,997, which is incorporated herein by reference. 
The preferred thermoplastic substrate for use herein is a polycarbonate 
film or sheet. Suitable polycarbonates may be prepared by reacting a 
dihydric phenol with a carbonate precursor, such as phosgene, haloformate 
or a carbonate ester. Typically, they will have recurring structural units 
of the formula: 
##STR11## 
wherein A is a divalent aromatic radical of the dihydric phenol employed 
in the polymer producing reaction. Preferably, the aromatic carbonate 
polymers have an intrinsic viscosity ranging from 0.30 to 1.0 dl./g. 
(measured in methylene chloride at 25.degree. C.) By dihydric phenols is 
meant mononuclear or polynuclear aromatic compounds containing two hydroxy 
radicals, each of which is attached to a carbon atom of an aromatic 
nucleus. Typical dihydric phenols include 
2,2-bis-(4-hydroxyphenyl)-propane; 
2,2-bis-(3,5-dimethyl-4-hydroxyphenyl)-propane; 
4,4'-dihydroxy-diphenylether, bis(2-hydroxyphenyl)methane, mixtures 
thereof and the like. The preferred aromatic carbonate polymer for use 
herein is a homopolymer derived from 2,2-bis(4-hydroxyphenyl)propane, 
i.e., bisphenol-A. 
The laminates of the present invention facilitate the melt bonding of 
layers of the high temperature material layers and specifically facilitate 
the twin sheet forming process of the present invention. 
Laminates for use in the twin sheets forming process of the present 
invention have a high heat distortion temperature thermoplastic material 
layer and a low heat distortion temperature thermoplastic material layer. 
Suitable laminates include those set forth in Hirt, Jr., et. al., U.S. 
Pat. No. 4,737,414, which is incorporated herein by reference and which 
discloses laminates having a polyetherimide layer and a layer selected 
from the group consisting of polycarbonate resins, polyestercarbonate 
resins and polyester resins. 
The multilayer laminates are prepared in the usual manner, that is by 
lamination of separate layers extruded in the normal fashion, or by 
coextrusion through a diehead whereby the layers become 
intra-die-laminated. Coinjection molding can also be employed. The 
thickness of each of the layers can vary widely and depends upon the 
specific application of the multilayer composite. Generally layers of 
thickness 50 mils or less can be used for the low temperature materials, 
preferably 3 to 20 mils, most preferably 10 mils. The high temperature 
material layer can have a thickness of generally 100 mils or less being 
usable, preferably from 30 to 90 mils, most preferably 60 mils. The 
laminates most preferably have a total thickness of from 70 mils to 80 
mils. 
The twin sheet formed hollow articles may be made by creating a layer of a 
first high heat distortion temperature thermoplastic material, a second 
high heat distortion thermoplastic material layer and a low heat 
distortion temperature material layer disposed between the first and 
second high heat distortion thermoplastic material layers, by either 
employing a single layered sheet of a high heat distortion temperature 
material and a laminate having a high heat distortion temperature material 
layer and a low heat distortion temperature material layer or preferably 
by employing a pair of such laminates or a pair of single layered sheets 
of high heat distortion temperature material and a film of low heat 
distortion temperature material. In any of these cases, the low heat 
distortion temperature material layer should be located between the high 
heat distortion temperature material layers during the twin sheet forming 
process to achieve bonding between the layers of high heat distortion 
temperature materials. The bonding is preferably achieved at the 
peripheries of the high heat distortion temperature layers.

DETAILED DESCRIPTION OF THE INVENTION 
As illustrated in FIG. 3, the preferred twin sheet thermoforming equipment 
10 of the present invention has two mold halves, preferably female mold 
halves 12, 14 as shown in FIGS. 1 and 2. The mold shapes set forth are 
exemplary, with other mold shapes also being suitable for the process of 
the present invention. For example, the mold shapes can be such that the 
finished parts are circular or oval in cross section. The upper mold half 
12 is movable from an open position away from lower mold half 14 as 
depicted in FIGS. 3 and 4, to a closed position adjacent lower mold half 
14 as depicted in FIGS. 5 and 6. Upper mold half 12 has a rectangular 
horizontal outer rim 16 which frames the outer periphery of the upper mold 
half. An inner section 18 slopes upwardly and inwardly from the rim 16 to 
an upper and inner horizontal mold region 20. The mold halves 12, 14 are 
preferably made of cast aluminum. Region 20 has holes 22 drilled 
therethrough to allow air to escape from the mold halves 12 during 
thermoforming. Preferably a vacuum is pulled through holes 22. A half 
cylindrical channel 24 is located in the rim 16 and extends from the 
exterior edge 26 of rim 16 to the interior edge 28 of rim 16. The lower 
mold half 14 is preferably similar in shape to the upper mold half 12. The 
lower mold half 14 has a horizontal outer rectangular rim 30 which frames 
the periphery of the lower mold half 14. A half cylinder channel 32 is 
located on the upper portion of rim 30 and extends from the outer rim edge 
34 to the inner rim edge 36 of rim 30 and is parallel with the channel 24 
when mold halves 12 and 14 are clamped together. An inner section 38 
slopes downwardly and inwardly to a lower horizontal rectangular mold 
region 40. The mold region 40 has holes 42 drilled therethrough to allow 
air to escape from the mold half 14 during thermoforming. Preferably a 
vacuum is pulled through holes 42. Inlet tube 44 for the high pressure gas 
used during thermoforming, may be supported for (i) vertical movement 
along and (ii) rotational movement about the longitudinal axis of vertical 
support rod 46 by attaching the tube 44 to the rod 46 by a hollow collar 
48. The inlet tube is preferably made of a metal such as aluminum. During 
thermoforming the channels 24, 32 clamp around tube 44 which extends into 
inner cavity 50 formed by mold halves 12 and 14. During thermoforming, 
high pressure gas, air is suitable, is forced from air source 52 through 
hose 54 to tube 44 and into cavity 50 between a top sheet 56 and a bottom 
sheet 58. Preferably both of the sheets 56, 58 are laminates, such as 
laminate 59 shown in FIG. 7, having a layer 60 of high heat distortion 
temperature thermoplastic material and a layer 62 of low heat distortion 
temperature thermoplastic material. In the process of the present 
invention the sheets are heated to a temperature above the heat distortion 
temperature of the HHDT material but preferably at a lower temperature 
than 50.degree. F. below any melting temperature of the HHDT material. If 
the laminates are used in the process, then the temperature of the 
laminates during thermoforming may be raised to above the melt temperature 
of the LHDT material because surface tension will keep the LHDT material 
on the surface of the HHDT material. The sheets are then placed in the 
open equipment 10 between the mold halves 12, 14 with the tube 44 inserted 
between the sheets, the mold is closed clamping the tube between the 
channels 24, 32 and clamping the sheets 56, 58 together at their 
peripheries between the rims 16 and 30. The mold halves may be at a 
temperature above the melt temperature of the LHDT material to further 
promote melt lamination between the sheets at a temperature below the heat 
distortion temperature of the HHDT material. Preferably, the mold halves 
have temperatures below the heat distortion temperature of the HHDT 
material. 
The sheets used may both be laminates as shown in FIGS. 4-7 wherein the 
laminates have a layer of HHDT material 60 and a layer of LHDT material 62 
laminated together in direct adhesive contact with each other. 
As shown in FIG. 8, the structure 64 obtained from the process of the 
present invention is hollow and rigid and has an upper shell 66 of HHDT 
material and a lower shell 68 of HHDT material wherein the upper shell and 
lower shell are adhered together by a LHDT thermoplastic material. 
EXAMPLES 
The following examples illustrate the present invention but are not meant 
to limit the scope thereof. 
TABLE 1 
______________________________________ 
The following examples illustrate the bond strength obtained 
employing the process of the present invention for 
polyetherimide/LHDT layer/polyetherimide.sup.c structures: 
Mold Bond Strength 
Example 
LHDT Layer Thickness 
Temp .degree.F. 
(lb/in) 
______________________________________ 
1 10 mil polycarbonate.sup.b 
330 280 
2 10 mil PBT.sup.d 
330 430 
.sup. A.sup.a 
0 330 150 
______________________________________ 
.sup.a Example A is a comparison example employing no LHDT bond layer. 
.sup.b The polycarbonate layer was a polycarbonate made by reacting 
bisphenol A with phosgene and had a heat distortion temperature of 
270.degree. F. 
.sup.c the polyetherimide employed was a polyetherimide having an HDT of 
360.degree. F. 
.sup.d PBT is polybutylene terephthalate.